1. Can’t add the App - Solution 3

Can’t add the add-in - Solution 3

You might need to be a site collection administrator

Even if you are a global administrator it is worth trying to assign yourself a site collection administrator to verify this isn’t the issue by selecting the site collection in question from SharePoint Admin Center -> Site Collections or by going to https://***-admin.sharepoint.com/_layouts/15/online/SiteCollections.aspx where *** is the name of your SharePoint domain.

Select the site collection in question and click on Owners -> Manage administrators.