Коли тобі було весело? = When did you have fun?

Коли ти почувався щасливим? = When did you feel happy?
IMHO

"коли ти був щасливий" - чому не прийняло?

I got desperate and tried the literal translation. Didn`t accept it. However, there is nothing wrong in Коли ти відчував себе щасливим?

This should be accepted: "коли ти почувався щасливим"

This program sucks. Everybody here has complained that happy is not весело but щасливий/ва. Yet no one at Duolingo seems to be paying attention. Бути весело means to have fun, have a good time. To be happy is an internal state, щасливий.