2016 Freightliner Cascadia 0 codes but Check Transmission light on.

Truck has 525K miles.
Bought it last week and when on the road, drove about 60 miles, check transmission light and HSA (Hill Start Assist) came on. Pulled over to shoulder and turned off thinking resetting would help. After turning it off and on, it beeped asking to be set on neutral to turn on the truck. Switched back and forth to Drive back to Neutral to Rev to Neutral several times didn’t help. The pressure in tanks were 130 or more so air pressure wasn’t the issue. And it is showing 0 codes in diagnostics tab.
So towed the truck to shop. After 2 days mechanic calls and says there is nothing wrong. He hooked it up to computer and it didn’t show any codes.
Yesterday I did loops around town and it was working fine. Decided I would go on my way home (1000 miles) when same thing happened.

So, any advice what could it be? What to check?