如果简单变量在另一个阶段中为空，请尝试as in here，即env var替代项：

def env.projectsString = ''
...
env.projectsString = ${projectsString} + f.name + "\\" + "n "
...
choice(name: 'CHOICES', choices: "${projectsString}", description: 'Please Select One')

要在多个阶段中可用，变量需要是环境变量：

environment {
    VARIABLE = "initial value"
}

并且您对它所做的更改必须采用此格式才能在其他阶段中使用：

env.VARIABLE = "new value"

在您的情况下：

pipeline {
    agent any

    environment {
        projectsString = "initial value"
    }

    stages{
        stage('debug') {
            when {
                branch 'master'
            }

            steps {
                script {
                def files = findFiles()
                files.each {
                    f ->
                    if (f.directory) {
                        env.projectsString = projectsString + f.name + "\\" + "n "

                        echo projectsString
                    }
                }
                }
            }
        }

        stage('debug2') {
            when {
                branch 'master'
            }
            input {

                message "Choose"
                ok "Submit"
                parameters {
                    choice(name: 'CHOICES', choices: "${projectsString}", description: 'Please Select One')
                }

            }

            steps {
                script {

                echo "Selected choice is : ${params.CHOICES}"
                }
            }
        }
    }
}

有关https://www.jenkins.io/doc/book/pipeline/jenkinsfile/#using-environment-variables详细信息，请访问www.example.com和https://www.jenkins.io/doc/pipeline/tour/environment/。