To start, please pick the appropriate category next time. If you’re having trouble managing your website, that’s a support question, so please post it in the Hosting Support category, not the Informal category.
I’m not completely sure, but I think the issue is that WordPress is unable to verify whether the changes don’t crash your website because of this system:
Unfortunately, there isn’t really a “fix” for this. A web search points me to a few possible solutions:
- Accept that you can’t use the built-in editor of WordPress, and edit your file using our web based file manager instead, or an FTP client of your choice.
- Modify the WordPress core code to remove this check. However, be aware that this may be removed when WordPress is updated, and you risk breaking your site completely if you do it wrong.
- Use a plugin that lets you edit WordPress files (but doesn’t do this check) instead.