Gitlab merge conflict button don't show

Question

I try to manage a merge request from a branch to a master branch using GitLab.

Until some days ago, when conflicts rise I can manage them with GitLab UI, using button Resolve merge conflict, but now I've only a button named Merge locally.

I don't know if this an issue of GitLab o I must change some settings to re-appear the button about resolution of conflict.

The Gitlab edition is CE.

EDIT

I've tried to remove protection from master branch but the button don't show.

I've tried to change the URL adding /conflicts in the path but the conflicts cannot be resolved.