It sounds like the .accdb file has become corrupted. If your granddaughter has not yet created any objects she wishes to preserve, then the simplest option is to delete the file and start again with a new file.
If, on the other hand, she has objects she wants to preserve, then she can create a new empty .accdb file, and from the External Data ribbon elect to import from an Access database, and browse to the original file. Then select all objects in the tabs of the dialogue form, apart from Modules, and import them into the new file.
Hopefully the new file will then perform as expected, in which case the original file can be deleted, and the new file be renamed if necessary.
If the same behaviour is experienced in the new file, MS 365 can be reinstalled by logging into her Microsoft account, and clicking on the Install button on the Subscriptions page.
PS: On re-reading your original post, it sounds like no .accdb file has yet been created, so repairing or reinstalling MS 365 will be necessary.