Declarative Pipeline with dynamic matrix axis values
Dynamic axis values are not supported in Declarative Pipeline as of v1.6.0 of the plugin. The error message says it expects a "literal string" value. Perhaps the message should also say, "Template strings, arrays, variables, or function calls are not allowed."
This is by design, intended to keep pipeline authors from getting bitten by some internal assumptions of the pipeline engine. It might be possible to change but will require careful planning and testing.
Please file a JIRA at https://issues.jenkins.io/ and/or join the Jenkins Pipeline Authoring SIG and help implement this feature.