The water supply line is stuck and the nut is removed. It turns with some force but won't detach from the bottom of the tank
That supply line should be disconnected at the shutoff valve as well and should then be fairly easy to bend and remove. The plastic nut at the base of the tank can then be removed to change out the fill valve.
You'll want to replace that supply line with a flexible line just a little longer than the distance from the shutoff valve to the base of the tank. If your shutoff valve is old and squirrely you might want to replace that as well with a ball and valve 1/4 turn.
Usually there's enough play up and down to remove the fixed supply line but some I've had to bend a little to remove. You'll probably want to replace it anyway.