GMC 3.0 Duramax DEF Empty Refill Now screen 

I believe it's an EPA thing that they can't let you dismiss the message if the DEF level is 0%. But I could be wrong about that. If you actually never got the DEF low warning, which I think is either at a half gallon remaining or when it drops below 1k mile DEF range, I would be suspicious. Last time I was towing with mine (2020 LM2), I got the warning popup with ~850mi range. Put my back seat spare jug in at a rest stop and went on my way. If I was in your situation at the moment, I'd dump a 2.5g jug in it and take it into the dealer. There's a TSB (#22-NA-150) for all 2020+ 3.0's and 6.6's which is for a reductant control module recalibration. In this case, I would make sure to not completely fill the DEF tank since it seems like the TSB is hinting at the tech needing to add a gallon for and/or to verify the calibration. From what I've been reading and hearing, it sounds like the calibration issue could be giving false or no data at low tank levels. I haven't seen anything like this with mine, but honestly I've never let my DEF tank get to a gallon or less.

I bet if you put DEF in it that will go right away