Doh, added this as a comment not answer, I’ll try again.
I had the same issue after removing one of the rear wheels. I tried purging the transmission but to no avail. I downloaded the manual for my mower - a Husqvarna CTH150XP. It ends up there is a 'square key' on the back wheels, that is easily missed when you remove the wheel. It will sometimes drop off onto the ground - often the lawn - and be lost or hard to find. You then replace the wheel with the circlip / washer etc. and the wheels don't turn. This is because the axle on the changed wheel is turning inside the wheel, and without the square key, the wheel is not turning when on the ground - note: both wheels will turn if you lift the mower as there is no resistance.
Search the internet for Husqvarna (or your make) square key to see what it looks like. Could this be your problem too?