The metal insert(s) in your desktop or shelf top may be too deeply recessed.
If an insert isn't flush with the surface, the screw won’t engage 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 surface. Repeat with other inserts as necessary. Once they are all level, you should be able to attach the top without issue.
For Bamboo tops (including for the Jarvis Desk Shelf), use a 6mm hex key. For Laminate, use a 7mm hex key.