Diagnosis
Some very slight flex is normal at standing height, but noticeable wobble at sitting height usually means the frame isn't square or the leg braces aren't tight enough. It's a common assembly tweak.
How to Fix It
Don't panic, you probably don't need to fully disassemble it. Lower the desk to its lowest height. Get your Allen wrench and check every single bolt you tightened during assembly. Focus on two key areas: 1) The bolts that connect the crossbar (the long bar with the foot) to each side frame—these are critical for rigidity. 2) The bolts that attach the metal side frames to the wooden desktop itself, especially the ones in the middle of each side. Tighten them all firmly in a criss-cross pattern. Finally, ensure all four feet are firmly touching the floor on a level surface.
If That Didn't Work
If the wobble is mostly front-to-back, the issue might be in the column mounting. The bolts holding each vertical lift column to its side frame might be loose. You'll need to access these from underneath. Tighten them while the desk is on the floor to avoid shifting the alignment.
Heads Up
The desk will always be most stable at its lowest height and have some inherent give at the top of its range—that's physics with a two-leg design. If you have carpet, consider putting a firm plastic mat under the feet to prevent a slight "spongy" wobble.