Frost forming inside the freezer indicates an air leak somewhere.
Top freezer door leaking.
The most likely place is around the freezer door gasket.
Open the door and place one end of a dollar bill at the top left corner of the door and close the door.
If the freezer door is not aligned properly get another person to hold the door aligned while another person tightens the hinge bolts hardware.
Use a towel to dry the seal and the surrounding areas and close the freezer.
To confirm the gasket is sealing correctly you can perform the dollar bill test.