I won´t pretend to be an expert but here are my thoughts. Is the clutch fully releasing? Pull the handle (engine off) and see if the bike rolls. If not, check out the clutch cable or R&R the clutch and check out the trans while disassembled. I am not familiar with the trans design so I cannot speak to internal component failure. I would start with a service manual and its diagnostics. Others may have better ideas. Good luck!