Check to see if the inserts are too deeply recessed.
If the metal insert isn’t flush with the surface, the screw won’t engage the thread and will instead appear to ‘spin’ around without attaching.
To fix this, take a hex key (not included) and “back out” the metal insert using a counter-clockwise motion until it’s flush with the desktop. Repeat with other inserts as necessary. Once they are all level with the desktop, you should be able to attach the frame to the top without issue.
For Bamboo tops, use a 6mm hex key. For Laminate, use a 7mm hex key.