anhducbkhn are you still experiencing the issue?
You're referring to the "start discussion" composer not offering you any tag?
Some troubleshooting steps:
Do you see tags on the forum sidebar on the homepage?
Do you see a javascript error in the browser console when clicking on "Choose tags" in the composer?
Does your user has a confirmed email and is not suspended?
Try disabling other third-party extensions to see whether the problem fixes itself.