How can I fix that so I can be tagged in the post?
Are you under 18? If so, you can only be tagged by friends or friends of friends.
According to Facebook Help Center's Who can tag me?
Anyone can tag you in photos and other posts. Tags from people you're not friends with may appear in your timeline review where you can decide if you want to allow them on your timeline. Remember, posts you choose not to allow on your timeline may appear in News Feed and elsewhere on Facebook.
Note: Minors can only be tagged by friends and friends of friends.