Diagnosis
That's the anti-tip sensor freaking out. The heater thinks it's not sitting level or upright, so it's refusing to operate as a safety precaution.
How to Fix It
First, just pick the whole unit up and set it back down firmly on a flat, hard surface. The little plastic feet all need solid contact. If that doesn't stop the beeping, unplug it and flip it over. Check that the little black tilt switch (a small plastic cylinder usually near the back) isn't stuck. You can gently tap the housing near it to try and free it up. Plug it back in and try again on a level floor.
If That Didn't Work
The sensor itself might have failed. If the heater is perfectly level and the beeping persists, the internal tilt switch is likely faulty and would need to be replaced, which isn't really a user-serviceable part on this model.
Heads Up
Never try to use it on carpet or a rug if it's beeping—that's it telling you the surface is too unstable. Forcing it to run in that state is a genuine fire risk.