I am having trouble shutting off water supply to toilet can I still replace the flapper?

Yeah... it takes all of about 30 seconds to replace it... you let the water run when you do it.

If necessary tie the float up while you work.

Both good answers, I would suggest to replace the Toilet angle valve just close the main S/O valve to the house and replace the angle valve, good luck.

John

As stated, you may hold the float in a off position or shut the water off at the main shut off. Let's talk about that sticky angle stop. And
Shut the water off at the house may and unscrew the bonnet back in not off the angle stop. Now take some plumbers heatproof grease and grease the treads reassemble the stop and see if that doesn't work better. Good luck, Tom