"The API has changed early this year (2020) when Google introduced shortcuts:
You can only add a parent for an item that doesn't already have one. This affects the children.insert (v2), files.update (v2 / v3) and parents.insert (v2) endpoints. You can use the new canAddMyDriveParent capability to check if an item currently has no parents and if the user has sufficient access to add a parent for the item.
This was announced in March but it's been implemented since September 30. "
I hope this helps with fixing this issue. Fingers crossed! <3
And thank you! I am super grateful for Doctopus! <3 It's been such a great help! <3
Kindest regards,
Rita