No "accept merge request" button with developer level access
As has been stated previously, the developer role is not allowed to merge by default.
Thus, change the Settings > Repository > Protected Branches > Protected branch table where the Allowed to merge table cell has the correct role, user, etc.
Developers can accept merge requests. However, it depends on how the project is configured, too.
Developers can accept merge requests when:
- The merge target is not a protected branch.
- The merge target is a protected branch, if an owner/maintainer has checked the 'Developers can push' checkbox on the protected branch setting.
If a developer is seeing the message you describe it is probably because of a protected branch and the 'Developers can push' box is unchecked.