Reference the Microsoft.SQLServer.Types dll from the project and for the reference set it as "Copy Local" in the properties of the reference. This will package that DLL up with the website when you deploy it. Then you don't need all of SQL Server installed on your web box in order to use the SQL Server data types. I did this for my website because it was using the geography data type columns and I was getting the same error.

I tried to resolve this issue by adding the Microsoft.SqlServer.Types NuGet package to my project, but that alone did not help. I also had to add the following to the <assemblyBinding> element of my project's App.config:

<runtime>
  <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
  .
  .
  .
    <dependentAssembly>
      <assemblyIdentity name="Microsoft.SqlServer.Types" publicKeyToken="89845dcd8080cc91" culture="neutral" />
      <bindingRedirect oldVersion="0.0.0.0-14.0.0.0" newVersion="14.0.0.0" />
    </dependentAssembly>
  .
  .
  .
  </assemblyBinding>
</runtime>

The solution that works for me is add "Type System Version=SQL Server 2012" to connection string.

Example: string connectionString = "Data Source=myserver;Initial Catalog=mydatabase;User ID=sa;Password=*******;Type System Version=SQL Server 2012;";

Rather than changing your project and adding a reference to Microsoft.SQLServer.Types dll you could just put it into the GAC.

In my case I had three versions on my dev machine and added them to the server GAC:

gacutil.exe -i .\10.0.0.0__89845dcd8080cc91\Microsoft.SqlServer.Types.dll
 gacutil.exe -i .\11.0.0.0__89845dcd8080cc91\Microsoft.SqlServer.Types.dll
 gacutil.exe -i .\12.0.0.0__89845dcd8080cc91\Microsoft.SqlServer.Types.dll

I had other SQL related third party .NET tools on the server that had the same error. After adding the assemblies to the GAC, they all worked fine.

While I don't put my own assemblies into the GAC, I think it is okay to put some Microsoft SQL-Server assemblies there because they affect multiple applications.

After other solution if you still have error try to delete in web.config

<add assembly="Microsoft.SqlServer.Types, Version=12.0.0.0, Culture=neutral, PublicKeyToken=89845dcd8080cc91"/>

copy file Microsoft.SqlServer.Types.dll to bin folder (or/and add reference)

or/and add "Type System Version=SQL Server 2012;" in sqlconnectionstring

For those experiencing this problem with PowerShell, I was able to fix my problem by installing the SQLSysClrTypes.msi package from Microsoft for SQL Server 2016 and restarting powershell. The download page is confusing to navigate, click "Download" and search the page for SQLSysClrTypes . Select the right architecture.

Exception calling "Fill" with "1" argument(s): "DataReader.GetFieldType(24) 
returned null."
At MyScript.ps1:15 char:5
+     $adapter.Fill($ds) | Out-Null
+     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], 
ParentContainsErrorRecordException
    + FullyQualifiedErrorId : InvalidOperationException

In my case, the offending data type was Microsoft.SqlServer.Types.SqlGeography in position (24) This field worked fine inside AppVeyor with SQL Server 2016 installed, but wouldn't run on my local environment.

I found several articles explaining why this occurs and some specified to use NuGet to fetch the latest Microsoft.SqlServer.Types.dll , but in my case, this didn't help, nor did any attempt to replace the assembly\GAC_... version with one from Install-Package and friends.

Note: Installing the .msi wasn't as straight forward as one would expect because through trial and error, I had installed several older "Microsoft SQL Server System CLR Types". This resulted in the MSI only offering "Repair" and "Remove". If this occurs, chose "Remove" and the run the installer again.

Installing the correct version and restarting PowerShell did the trick.

For me had to add 10.0.0.0__89845dcd8080cc91 folder inside it added the
Microsoft.SqlServer.Types.dll

for version 10

to C:\Windows\assembly\GAC_MSIL\Microsoft.SqlServer.Types

Probably there's hidden dependency that uses GAC instead of the the project directory

Using (the oldest) version 10.50.1600.1 fixed this for me.