This isn't a project infrastructure issue, so moved it to the JENKINS project, and assigned it to the correct plugin.

Also, I don't know whether you attached the correct Jenkinsfile. When I download the attachment I see a very bare pipeline with no reference to environment variables or git.

I just uploaded a new Jenkinsfile, which should illustrate the issue described.

Ok, the second attached Jenkinsfile makes more sense. This is happening because the variable reference is ambiguous.

In the step git "git@example.org:user/$REPO.git", the variable name is being interpreted as REPO.git, and so Jenkins tries to access the git property of the REPO object, which isn't possible, as REPO resolves to a string.  Hence the error message you saw.

If you use git "git@example.org:user/${REPO}.git", then it will work as expected.

Perhaps the declarative pipeline plugin can do a better job of detecting such ambiguities, so I'll leave this open, but you should have no problems so long as you ensure that your variable references are unambiguous.

Thank you for clarifying my mistake. Would you say it better practise to always use ${} syntax instead? This seems more fool proof.

> Would you say it better practise to always use ${} syntax instead?

Yeah, that's certainly what I always do within a double-quoted string.

The documentation gives an example with that style when discussing string interpolation, though I don't know if that style is used consistently throughout the rest of the docs:
https://jenkins.io/doc/book/pipeline/jenkinsfile/#string-interpolation