error: You are not allowed to push code to protected branches on this project.
...
error: failed to push some refs to ...

there‘s no problem - everything works as expected.

In GitLab some branches can be protected. By default only ‘master‘ user can commit to protected branches. master branch is protected by default - it forces developers to issue merge requests to be validated by project master before integrating them into main code.

You can turn on and off protection on selected branches in Project Settings (where exactly depends on GitLab version - see instructions below).

On the same settings page you can also allow developers to push into the protected branches. With this setting on, protection will be limited to rejecting operations requiring git push --force

I‘m not really sure when this change was introduced, screenshots are from 10.3 version.

Now you can select who is allowed to merge or push into selected branches (for example: you can turn off pushes to master at all, forcing all changes to branch to be made via Merge Requests). Or you can click "Unprotect" to completely remove protection from branch.

source：

https://stackoverflow.com/questions/32246503/how-to-fix-you-are-not-allowed-to-push-code-to-protected-branches-on-this-proje

Git you are not allowed to push code to protected branches on this project?