Thanks for getting in touch! How is this variable scoped? There are some restrictions to binding the Release Versioning in your project to a variable. Only unscoped variables or variables scoped only to a channel can be resolved at release creation to define the release version.
You can also define the release versioning to Use the version number from an included package which will keep release versions in tandem.
Let me know how you go and if you have any further questions going forward!