Is this a modification of GitHub - ajnyga/forthcoming: OJS3 plugin for showing forthcoming articles ?
If so, you will want to fork the original plugin on GitHub from ajnyga’s repository to your own, rather than copying the files and re-adding them in GitHub. This helps to track changes and facilitates merging code back into the original repository.
To do this, use the “Fork” button in the upper right of GitHub - ajnyga/forthcoming: OJS3 plugin for showing forthcoming articles to create a fork in your user space. Then, clone your forked repo, and copy the files you’ve been working on into that clone. You’ll then be able to commit the changes and we’ll have a clear picture of what has been modified and we can advise you on why the save isn’t working.