There are a few causes of this problem.
It's a fault on the tachometer circuit of the motor.
The problem can be the motor, control board, the wires to the tachometer or the connection at the control board.
Unplug the appliance.
Take the rear panel off and gain access to the motor and motor wiring.
(You'll need torx drivers or bits for the screws).
Check the thin wires at the far end of the motor (not the belt end) and see if they have broken or if the tachometer magnet has come out.
Check the wires at the motor plug corresponding to the thin ones.
Check the same at the control board (you'll need the top off to check the facia board).
If all appears ok, you have a fault with the motor and/or control board.
Obviously, if you cannot do the work yourself, you'll need to get someone else to do it for you.
Hope this helps