If you came here and nothing worked, don't worry, it's not your fault.
You probably have executed these two commands to install the plugin as per the official documentation:

$ ionic cordova plugin add <your plugin>
$ npm install --save @ionic-native/<your plugin>

You have also added the plugin in the app-module.ts file:

@NgModule({
    ...

    providers: [
        ...
        Your plugin
        ...
    ]
...
})

You are already making calls to your plugin which has been correctly imported and injected into your calling class's constructor.
You are even waiting for the deviceReady event to start using your plugin:

this.platform.ready().then(() => {
    //Use plugin now
});

And then you are still getting the plugin_not_installed error. One thing that could be happening is that, despite this multi-MB clusterfuck of node and configuration files, the plugin was added recently while the project was created some time ago. When you added the plugin it has downloaded the most recent version available in the repository (!!!) and for some of the platforms installed in your project (android or ios) this plugin needs a Cordova version greater than the one you have now. Type again the first command:

$ ionic cordova plugin add <your plugin>

And look carefully at the output. It looks like it went OK but if you scroll up you might find an error saying that this plugin you have downloaded requires Cordova android (or ios) version X and you have Cordova android (or ios) version Y with Y < X. Example:

Fetching plugin "phonegap-plugin-push@~2.1.0" via npm
Installing "phonegap-plugin-push" at "2.1.0" for android
Plugin doesn't support this project's cordova version. cordova: 7.0.2, failed version requirement: >=7.1.0
Skipping 'phonegap-plugin-push' for android

What is worse, the plugin has been partially added and it might be present in the root plugin folder and the config.xml , and is listed in the cordova plugin list command output, but it is not present in the platform_www\plugins folder.
If this is the case you need to update the offending platform. And cordova platform update has been deprecated, so you now need to do this:

ionic cordova platform remove android
ionic cordova platform add android@X

Where X is the version the plugin needs or greater, for instance "7.1.0".
Now you need to properly install the plugin again:

$ ionic cordova plugin add <your plugin>
$ npm install --save @ionic-native/<your plugin>

We are not done yet. In Android you might now get this error when running on device:

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:processDebugManifest'.
> Manifest merger failed : uses-sdk:minSdkVersion 16 cannot be smaller than version 19 declared in library [:CordovaLib]

After updating the platform, some of the new code requires a higher new minSDK. For ionic, you need to change this in the config.xml :

<preference name="android-minSdkVersion" value="19" />

And hopefully you will be OK now. Cordova really sucks in dependency management. Also the documentation is written as if everybody had the latest everything.

Use platform.ready() function before using the object of the imported class:

constructor(public qrScanner: QRScanner) {
                
                // solve the problem - "plugin not installed".
                platform.ready().then(()=>{
                  this.qrscanner();
                })
                
}

This is a puzzling problem because it is not detailed. According to my observation, it may be an incompatibility between the version of the android and the plugin. For my case, ionic had generated android@7.0.0 while the plugin in this case "phonegap-plugin-push" required version android@7.1.0. How to have this information? This information is displayed when you run the "ionic cordova build android" command with the error messages that appear. So I went around the problem by following these steps:

1. run ionic cordova platform rm android
2. do manually remove config.xml
3. run ionic integrations enable cordova --add to generate a new config.xml
4. do (if the plugins are not a lot,) reinstall them all
5. run ionic cordova platform add android@<version-android> if the version is required by the plugin.
   However, it may be necessary to reinstall all the puglins to make sure everything is in order. How to reinstall a plugin properly?

REINSTALL A PLUGIN 2 STEPS
Uninstalling two commands:

1. ionic cordova plugin rm <plugin name>
2. npm uninstall --save <npm-of-plugin>

installing two commands:

1. ionic cordova plugin add <plugin name>
2. npm install --save <npm-of-plugin>
   I hope this approach will help you to continue your projects properly. In any case, it worked for me.

IMPORTANT TO KNOW

For my previous intervention, I think it describes the solution to reinitianilize the ionic application by verifying that all plugins are installed. Because my application had bugs on a real device, no request to the API was working, and it was linked to a plugin that did not get well installed in this case the plugin "Document-viewer". While the error on the console of "DevApp" was related to the plugin "push-notification" and not the plugin "Document-viewer". And yet between these two plugins it was the plugin "Document-viewer" which was not installed well and the plugin "push-notification" was installed without problem. The proof, when I do the reset of the application by reinstalling all the plugins, my requests to the API works perfectly without bugs on a real device including notifications that work while on "DevApp" I always have the error "plugin_not_installed" of the push.
In conclusion , I think that trying to reinstall the plugin does not solve the problem in any case, because I think that it is the test applications that do not support all the plugins as was the case here with "DevApp".
Solution So try to launch your application on a real device if it's bugs, tell you that all the plugins are not well installed.

我也遇到了这个问题。问题是插件的版本不匹配。解决办法是通过shift + delete和package-lock.json删除node_module/。用npm i重新安装节点模块，然后用ionic cordova platform remove android删除平台，然后添加平台并安装所有需要的插件。

我对IBeacon插件也有同样的错误。试着用xcode构建你的应用程序，并安装在你的iPhone上。
我认为问题是cordova对于一些插件（如IBeacon）。当我用ionic serve -c运行项目，并在我的iPhone上用Ionic DevApp打开它时，cordova返回错误plugin_not_installed。
所以我尝试在xcode上使用命令ionic cordova build ios --prod构建应用程序，启动在/platform/ios上生成的.xcworkspace文件，并将其安装在iPhone上。（记住在xcode中签署应用程序并更改捆绑包标识符）！
有关https://ionicframework.com/docs/intro/deploying/的更多信息

不得不删除并重新创建Android文件夹，现在它正如预期的那样工作。就这么简单。
我希望这对其他开发人员有用