Using CI/CD Variables and include in gitlab-ci.yml

Hello,

is it not possbile to use CI/CD-Variables that have stored a path to a project and a filename in an “include” ?

Like this:

include:
- project: $PATH_TO_PROJECT
  file: $FILENAME

If i do so, Gitlab instantly shows me this in the CI/CD editor:

This GitLab CI configuration is invalid: Include {"project":"","file":""} needs to match exactly one accessor!.

How can i make this work?
Thanks in advance!