Trying to buy general admission and fast pass for this Sunday but the general admission is grayed out however, the fast pass isn’t. Am I able to buy a general admission for any day and go Sunday if I have the fast pass ?

no, if it's grayed put you can't go that day. fast passes may be available for the people that already have reservations but with out a reservation you can't get in

This year they require reservations to be made before hand. If you buy a general admition ticket for that day, a reservation is automatically applied with it. If you have season passes you must get a reservation for each day you want to visit. Reservations do not cover both parks, if you want to go the water park as well, a seperate reservation for that must be made.