Diagnosis
The charcoal grate is not installed in the correct orientation relative to the support rails. The grate has a specific top and bottom side.
How to Fix It
Remove the charcoal grate. Inspect it. You will see one side has a more pronounced upward curve on the support wires. This is the top. The flatter side faces down. Place the grate so the curved wires rest on the support rails inside the lower charcoal bowl. It will sit securely and level.
If That Didn't Work
Verify the lower charcoal bowl support rails are not bent from shipping. They should be parallel and level. If bent, use pliers to gently straighten them.
Heads Up
Do not operate the grill with the grate unstable. Falling embers can prematurely ignite fresh charcoal or overheat the ash pan.