Hey, maybe these previous questions/threads could help!
You can follow this documentation if you have not looked at it yet
Usage of a regular expression, that you define in the project explorer, is only assignable to the validation-expression of an entity:
You have probably created such a regex and are now trying to use it/call it in a microflow. That is just not possible in Mendix. Your only option, as far as I know, is in the microflow in an expression call isMatch().