Using SharePoint Designer on the root site collection
Even if a user has full control permission on the root site, he may not have permission to add and save SharePoint Forms Designer Add-in to SharePoint root site collection. This permission behavior can vary from tenant to tenant. In order to check the user permission, open “Site Settings”, then select “Site Permissions” under “Users and Permissions” block. Here you can check the user or group permissions for the root site. If you can't add and customize pages, you can't add and save SharePoint Forms Designer to the root collection as well. This issue with add-in saving doesn't appear on sub sites.
In order to fix this permission issue, do the following steps:
“Add and Customize Pages” permission on the previous screenshot is from site level, not from list permission level. Even if you have full control in list permission level, you may not have the site level permission. Add a new permission level, that includes only the “Add and Customize Pages” permission, and create new SharePoint group with this permission level.
Then add yourself into the SharePoint group to get the “Add and Customize Pages” permission. You must have enabled Custom Scripting in SharePoint admin center. Go to SharePoint admin center> Settings> Custom Script.
Note: Any change to the scripting setting made through the SharePoint Online admin center may take up to 24 hours to take effect.