Please power down and remove (and inspect) both Y-axis belts — if they are in good condition, mark them so that when reinstalled they can be swapped end-for-end and side-for-side.
Check the pulley set screws: https://docs.carbide3d.com/shapeoko-faq/shapeoko-3-how-to-check-the-pulley-set-screws/
and the wiring and connectors
If everything is in good condition and secure, power up, connect to the machine and try intializing — the machine should home the Z-axis as normal, and begin moving both X- and Y-axes — the X-axis should finish homing, while the Y-axis motors should turn until they time out — do they turn evenly and in synch?
If they do, re-install the belts and re-tension per the instructions and see:
If you continue to have difficulties, let us know what you found from the above at support@carbide3d.com and we will do our best to assist.