Diagnosis
The crossbar is not tightened enough. This is the metal brace that connects the two legs under the desk. It is the primary source of lateral stability.
How to Fix It
Get a hex key set. Flip the desk on its side (get help). Check every bolt connecting the crossbar to each leg. Tighten them firmly. Then tighten the bolts securing the legs to the desktop itself. Do not overtighten into the particle board.
If That Didn't Work
The floor is uneven. The adjustable feet are only for minor leveling. If the wobble is a rocking motion, place a shim (a thin piece of hard plastic or folded cardboard) under the appropriate foot until the rock stops.
Heads Up
All two-leg standing desks have some inherent forward/back wobble at full height. Side-to-side wobble at low height means the frame is loose. Fix it now before bolts work themselves out completely.