There is no cause for alarm on this.
It appears that you have installed a plugin on your site and is affecting your permissions on the site.
Check back at the plugin that was installed before you noticed this error.
or
Create a backup of your site, deactivate all the plugins you read might be responsible. Test for the issue. Repeat until you have checked each plugin or until the file saves. This was my plan to find my solution.
Alternatively,
Theme Editor also works
or
You can use the File Manager plugin, it would solve your issue... go to the Plugins->Add New->Type File Manager in the search...
Good luck!
