It is may be due to technical glitch you need to wait that the issue is resolved in neet if you dont fill the choice locking option the choices provided and saved shall be auto locked after the choice locking period is over by the system.

Just try it repeatedly u will get through it. Also, make sure you visit the page at around 10 AM when traffic is less dont waste time in locking choices on peak timing ensure you should fill it before 20th August.