Diagnosis
Some minor front-to-back flex is expected at full height, but significant side-to-side wobble usually means the crossbar isn't installed correctly or the legs aren't square to the frame.
How to Fix It
Flip the desk on its side (carefully, protect your floor). Check the metal crossbar that connects the two legs—it should be bolted tightly to the inside of each leg bracket. The most common mistake is installing it upside down or backwards. The flat, pre-drilled plate should face up toward the desktop. Tighten all those bolts with the included hex key until they're seriously snug.
If That Didn't Work
If the crossbar is right and tight, the issue might be your floor. Put a level on the desk surface at a low height. If the floor is uneven, the whole frame is racked. You'll need to adjust the height of the individual feet (they screw in/out) to stabilize it, then re-tighten the lock nuts.
Heads Up
The stability is hugely dependent on a heavy, rigid desktop. If you're using a lightweight or flimsy aftermarket top (like a hollow-core door), it will amplify any wobble. Branch's own solid wood or laminate tops help a ton.