Try shifting the bike with the motor off. Put it on a box so the rear tire is off the ground as that is easiest. Then kneel down next to the bike on the shifter side and see if you can shift it. You may need to rotate the rear tire in the forward direction, which is why I recommend putting the bike on a stand so the rear tire is off the ground. You should easily be able to go through the gears with your hand and maybe a little rear tire rotation. Once that's proven to be functional, try sitting on the bike and doing the same thing with your foot. If successful, then start the bike's motor and let it warm up. Repeat trying the shifting by hand, but this time use the clutch with each shift. If that all works, then straddle the bike and try it with your foot. And finally, take the bike off the stand and ride for effect.
Some bikes are harder shifting until the oil is warmed up.