不能在单引号或三重单引号中插入变量。请使用三重双引号：

stdoutpowershell = powershell returnStdout: true, script: """
      write-output "Server is $env:serverName"
  """

传递变量有两个选项。

• 环境导出变量
• 局部变量

添加完整的脚本，请参见下文。

node {
    stage('Invoke Installation') {
        def stdoutpowershell
        def serverName = "env.fqdn"
        
        // Use Env export variable
        withEnv(["SERVER_NAME=$serverName"]) {
            stdoutpowershell = powershell returnStdout: true, script: '''
                write-output "Server is $env:SERVER_NAME"
            '''
        }
        println stdoutpowershell
        
        // Use local variable
        stdoutpowershell = powershell returnStdout: true, script: """
            write-output "Server is ${serverName}"
        """
        println stdoutpowershell
 
    } 
}