You see the error message: "Sorry, you don't have access" when opening a File Picker to add a document, an image etc. to your ShortPoint design element.
Actual result with the error message:
This issue might happen even if you are the Site Collection Admin.
You do not have permissions on the root site of your tenancy.
To be able to access the File Picker, you need at least read permissions on the root site.
Please, contact your Global Admin or IT department to share the rights with you.
After you have the rights on the root site, you will be able to open the File Picker again: