Being unable to be happy means being Miserable
Sad
Infertile is the word that means unable to have children.
"Illiterate" means unable to read or write.
A person who can not or will not speak is known as a 'mute'
The prefix "un-" could be added to the word happy to create the word "unhappy," which means not happy.
undoubtedly
Unable
greatful is another word for happy
HELLO!
Healthappy (healthy and happy)
Inimitable!
Inexplicable