Whenever accessing scm property from the Scripted Pipeline the below error is being thrown:

ERROR: ‘checkout scm’ is only available when using “Multibranch Pipeline” or “Pipeline script from SCM

Example code:

node {
  def tempSCM = scm
}

Please note that the checkout command was not used. Also, when assigning anything to that property no error is thrown:

scm =
        [$class           : 'GitSCM', branches: [[name: "*/$testsBranchName"]],
         //...
         credentialsId    : 'GitUser'
        ] //<- no error here

Attached system info