Solenoid clicks, engine doesn’t turn over

I have a 2 year old Cub Cadet with a 22hp Kohler engine. Out of the blue, the mower wouldn’t start. Solenoid clicks when I turn the key, battery voltage is good however there’s only 4 or 5 volts getting to the starter. I assume this is too low to crank the engine. Any suggestions or advice? Solenoid? Wiring? The flywheel is also rusty, i took off the engine cover off and the flywheel seems to be stuck as well. I’m hoping it’s not a seized engine.
Looking for advice, thanks!