TCL error code 12 after a clutch replacement?

I had a Mitsubishi Galant V6 -98 which skidded on the clutch.
I engaged a repair workshop for clutch replacement and spark plugs replacement, plus any gland, which would be replaced. When I drove the car to the repair workshop I had no problem to move into a higher gear. Then the problem was just that it skidded on the clutch so that I must be feather-light on the gas.
After the workshop changed the clutch and spark plugs they call me and say that the car is extremely slow and can’t move into a higher gear. They ask me if I had any problems with this before and I tell them that I have not had any such problems. I point out to them that the car had been standing still for some weeks and then perhaps this is the cause.

The car then returned to an authorized Mitsubishi service center for troubleshooting, with computer I assume. They told me after a while that the trottle sensor is bad and must be changed with a new one, but still the same error remains. Then they found another sensor that must be replaced. After that they replaced the whole control box, but the problem persists. TCL-error code 12 is valid and the engine revolution speed is maximized to 1800 rpm, even in stand-still position.
So far, I then have spent lots of money to get the car in driving condition.
I am skeptical as the above changing of sensors hasn’t solved the problem. So I am in desperate need of HELP.
My idea is that the clutch replacement has caused the problem.
Can incorrect use of clutch, faulty spark plugs, faulty cable connections to the spark plugs, faulty fuel pump, clogged fuel filter, etc. be the cause..