Welcome to BYC. Your chicken may need some grit. You can find grit at most feed stores. Fresh water should be available at all times. If your chicken is a female she will need oyster shell once she begins laying.
Along with everything else suggested above, make sure to store this feed properly, inside a somewhere where it will stay dry, clean and cool. Chicken feed spoils fast. Do not add anything to the feed.
Welcome to BYC! Could be your chicken is not eating because of depression due to no other chicken to fellowship with?
When one of my 2 chickens died the lonely one went insane until I got another chicken (or 2) I now have 3 lol She was SO happy when I got the other 2 chickens