Sounds to me like it is in limp mode. I would check the ATF level and condition, clear the DTC and see if it goes right back into limp mode.
And your original information you shared, slipping in 1st, is what the error shows. The TCM wanted 1st gear ratio, and it was detecting a higher-than-acceptable slippage during that command. So it goes to limp mode. According to my info, it is greater than 10% difference (so input speed vs. output speed) in expected for 12 times longer than 1000 ms (so, one second).
You'd have to look at a flow/apply chart to see what is being commanded on/off to make 1st gear. If it is JUST that gear, and no others, and the ATF is full and reasonable condition, it could just be a control side issue.
Edit: I see an apply chart was provided by TurboABA.... and you can see there that the K1 apply clutch is working for 1st through 5th ratios. Which would *possibly* mean the gearset/clutchsets are OK, and you have some sort of control problem. *However*, since the error happens for 1st, it may indeed be the K1 clutch is lunched and the TCM never lets it get into 2nd to find out. This is where the condition of the ATF can give some clues.