Translation:He plans to go to the gym in the evening.
'у фітнес-залі' is used in the sense 'to be in a(the) gym', while 'у фітнес-зал' is used to say that you are going TO the gym. I think somewhere in the course there should be a description of questions 'де?' and 'куди?'. 'у фітнес-залі' is the answer to the former, and 'у фітнес-зал' is the answer to the latter.
I wouldn't say it's wrong. Just that in English we can call pretty much anytime after dark "night". In Ukrainian night that part of the night when everyone should be sleeping that we might call morning in English (approximately between midnight and dawn) . Right now only the literal translation of evening is accepted, report your answer as correct and hope it gets added to the accepted answers soon.