Diagnosis
You have the legs reversed. The legs are side-specific but not always clearly marked. The wider stance is for the front.
How to Fix It
Lay the griddle body on its back on a soft surface. Identify the legs with the caster wheel holes; those are the rear legs. The plain legs are the front. Align the rear legs so the wheels are at the back. The front legs have a slight outward angle for stability. Start all bolts finger-tight before fully tightening any.
If That Didn't Work
The frame might have been bent in shipping. Check the squareness of the leg mounting brackets on the griddle body. Gently pry a misaligned bracket with a flathead screwdriver.
Heads Up
Do not force the bolts. Cross-threading the welded nuts in the frame is a major repair. If it won't start by hand, the alignment is wrong.