Freightliner cascadia not going into gear

Edit: Most importantly you want a Detroit Certified technician with the proper software to diagnose. With the misrepresentation when failed sensors or wiring shorts occur, we are only stabbing in the dark.
Will it go in Manual mode?

My two cents:
Likely the clutch is gone. The relearn process basically designates a distance or range to engage/disengage the clutch. If the wear is excessive it cannot function. And/or your TCM has failed. Typical checks entail wiring harness or air leak. Did a qualified mechanic do the relearn? If so, what did he think?
The problem with the AT12 is you have to drop the whole unit out to check and that alone is thousands of dollars. I have a 2016 Western Star and I had a similar failure at 780k miles and I had the clutch, clutch brake and actuator replaced. That was about $8500. And then 100k miles later it failed again. I ended up putting in a UTI Rebuilt, TCM, ECU shift positioner and that ran me $17,000US.

I got in touch with the rebuild manufacturer as I knew they would get my core and I wanted to know what they found. Among collateral damage they found the previous work had mostly failed. The brake and actuator failed. And the split shift pin, which was an original recall and they had checked, was laying in the bottom of the tranny. Third gear was worn to a smooth surface (since the clutch brake failed) but they still felt it was at the end of its life and wasn't to my benefit to pursue.

Depending on you mileage you'll want to weigh similar expectancy as these transmissions have a service life, they're highly complex and eventually the expensive process of replacing it is inevitable.

Best of luck

 

521001/14 for automatic mode not available - gear shift no plausible.

Be sure to check for any faulty fuses or relays as well as between your battery box and control module on this truck to confirm that all of your control modules are powering up and there are no voltage supply related issues existing. Check for any problems with the wiring or connector pins going to your control modules as well that would result in an open circuit and loss of data link communication. If all this checks out OK, you very well could have a software calibration related glitch requiring dealer level programming and updates in order to resolve this to reset any of the control modules back to factory settings with the most current updates in order to resolve any software related glitches.