Have you tried to see if you face this issue with other templates as well.
I had the same issue, and realized that the recommended security settings by my host management system was probably what was causing this issue.
I am using Plesk for one of my sites, and their recommended “Disable file editing in WordPress Dashboard” was probably the reason in my case.
In your case, the security plugin is what is causing the issue, which you can resolve by un-checking the Disable File Editor from Security -> Settings -> WordPress Tweaks -> Disable File Editor. https://themeskills.com/disable-wordpress-plugin-theme-editor/