Verify that your X axis pulley is correctly aligned and not touching the X motor.

The belt tensioner location will vary, depending which X axis you are using.

Bear X axis (BearExxa, BearMera, Bondtech on Bear X axis): tensioner is on the right of the X axis (X end idler).

Original Prusa MK3(S) X axis: tensioner is on the X motor arm.

Original Prusa MK2(S)/MK2.5(S) X axis: there is no tensioner, you can only adjust the tension of the belt on the back of the extruder.

Grip the X motor shaft with pliers to prevent it rotating (grab the flat side of the shaft).

Tension the X axis belt. Do this while trying to move the extruder to the left or right. The belt should stay straight and should not bow up and skip over the drive pulley teeth.

No need to be very precise with the belt tension here as the final tension will be improved later (unless you have MK2(S)/MK2.5(S) X axis).