Checksum Fail on full flash no B9999 param

I'm in dire trouble. I am working on a 2010 Chevy 2500 6.0L vin 1GC4KXBG3AF147376 OS:12636005.
I made a copy of the original tune. Then disabled the downstream O2s in the DTC type n mil. When I did the full flash, at the very end I got the checksum fail $06FF. Now the truck will not start
Nothing I do allows me to flash the computer and now 7.5 scan won't recognize the E38. There is no checksum fail parameter B9999.
Where do I find the checksum patch or how do I fix this. Thanks