If your line manager has created a folder for you to put your FlexSheet in it will almost certainly already be set up with the right access permissions. Just moving your FlexSheet into the folder will make it inherit those permissions, giving your line manager the access they need; you won’t have to do anything further.
You can check and, if need be, alter the sharing permissions of your FlexSheet to let your line manager see it. Sometimes this is done by moving your FlexSheet into a shared folder they have created; other times they might ask you keep your FlexSheet in your own My Drive area but share it direct with your line manager. Here’s how to do the latter:
If you accidentally give access to someone who should not have it just: