1998 Buick Lesabre

P0742 Torque Converter Clutch Circuit Stuck On
Hopefully just a short or a bad solenoid.
Quite often though it's caused by debris from damage in transmission that jams the solenoid or a valve.
I don't know if a can of trans conditioner would help or not, you could try it.
The first thing I would do is remove the transmission pan to see if there are any fine metal or brass filings there. You should also break open the filter and unfold the element to check for debris ( the filter is a great vacuum cleaner and can hide a pile of junk! )
If there is alot of breakdown then I'm afraid you are in for an overhaul or used trans swap.
But even swapping solenoids probably means trans removal.