The declarative pipeline comes with some limitations if it comes to its syntax. You see this error because in the environment block you can assign only two types of expressions:

• strings (single or double quoted)
• values returns from function calls

However, you need to be aware that the environment variables store only string values, so if you return an array (or any other type from) from the function call, it will be automatically converted to its toString() representation.

pipeline {
    agent any 

    environment {
        MYPODS = getPods()
    }

    stages {
        stage("Test") {
            steps {
                script {
                    println "My pods = ${env.MYPODS}"
                }
            }
        }
    }
}

def getPods() {
    return ['pod1', 'pod2']
}

Console output:

[Pipeline] node
[Pipeline] {
[Pipeline] withEnv
[Pipeline] {
[Pipeline] stage
[Pipeline] { (Test)
[Pipeline] script (hide)
[Pipeline] {
[Pipeline] echo
<java.lang.String@ed6c7b35 value=[pod1, pod2] hash=-311657675>
[Pipeline] echo
MYPODS = [pod1, pod2]
[Pipeline] echo
Item: [
[Pipeline] echo
Item: p
[Pipeline] echo
Item: o
[Pipeline] echo
Item: d
[Pipeline] echo
Item: 1
[Pipeline] echo
Item: ,
[Pipeline] echo
Item:  
[Pipeline] echo
Item: p
[Pipeline] echo
Item: o
[Pipeline] echo
Item: d
[Pipeline] echo
Item: 2
[Pipeline] echo
Item: ]
[Pipeline] }
[Pipeline] // script
[Pipeline] }
[Pipeline] // stage
[Pipeline] }
[Pipeline] // withEnv
[Pipeline] }
[Pipeline] // node
[Pipeline] End of Pipeline
Finished: SUCCESS

Solution

If you want to store a list of string values then you can define it as a single string of values delimited with , character. In this case you can simply tokenize it to a list of values. Consider the following example:

pipeline {
    agent any 

    environment {
        MYPODS = 'pod1,pod2,pod3'
    }

    stages {
        stage("Test") {
            steps {
                script {
                    MYPODS.tokenize(',').each {
                        println "Item: ${it}"
                    }
                }
            }
        }
    }
}

Output:

[Pipeline] node
[Pipeline] {
[Pipeline] withEnv
[Pipeline] {
[Pipeline] stage
[Pipeline] { (Test)
[Pipeline] script
[Pipeline] {
[Pipeline] echo
Item: pod1
[Pipeline] echo
Item: pod2
[Pipeline] echo
Item: pod3
[Pipeline] }
[Pipeline] // script
[Pipeline] }
[Pipeline] // stage
[Pipeline] }
[Pipeline] // withEnv
[Pipeline] }
[Pipeline] // node
[Pipeline] End of Pipeline
Finished: SUCCESS

your environment should be after the agent

pipeline {

agent any
environment {
    def mypods = []
}
stages {
    stage('Getting pods') {
        steps {
            script {
               withKubeConfig(caCertificate: '.....', credentialsId: '.....', serverUrl: '.....') {
                    env.mypods = sh "kubectl get pod | grep Running | awk '{print \$1}'"
                }
            }
        }
    }
    stage('Print pods') {
        steps {
            script {
                mypods.each {
                    println "Item: $it"
                }
            }
        }
    }
}

}