question about gitlab merge request (invent.kde.org)

Hello! Yesterday I submitted my first merge request to a KDE software! I forked the repo, applied the edits, committed them and pushed to my fork, and then from gitlab created a merge request. after a few hours, the author pushed something into the master, making my merge request 1 commit behind the master. On the merge request details page, I now get this error:

    
Merge blocked: the source branch must be rebased onto the target branch.  [rebase without pipeline]  [rebase]

  

If I try to click on [rebase], I get this red error:

    
Source branch is protected from force push

  

what should I do to make my merge request "mergeable" again?