Hi zoli10,
Based on the current situation, if you’re using Office 365 for Professional and Small Business, you need to guarantee you have got to top level site settings before editing the permission levels.
Please refer to the following steps:
1. Sign in to your SharePoint Team Site.
2. Click “Site Actions” -> “Site Settings”.
3. Click “Go to top level site settings” under “Site Collection Administration”.
After that, you can then go to “Site Permissions” -> “Permission Levels” to see if it works.
For more information about editing permission levels, I’d like to suggest you refer to:
Edit, create, and delete permission levels
Thanks,
Allen Zhang