Key is sheared!
It needs replacing!
Key-key fragments are possibly wedging motor sprocket on.
"Key" is solely to keep shaft and sprocket spinning together, not to hold sprocket on.
You might need gear puller to remove.
"Flaring tool" will work ... if you have one.
"Pliers" sideways, between motor and sprocket, if "worked around" sprocket, should work sprocket off.
Make sure to replace nut, it keeps the key solidly seated against shaft and sprocket. Otherwise it will impact against them, with every throttle use ... quickly shearing key.

48V might be too much torque for the key?
Use only partial throttle, especially at low rpms! Also reduces damaging heat