A word that means that something doesn't matter if not finished
Solution 1:
This isn't a literal translation of the concept (actually, 'moot' is kind of a hard word to explain), but I'd use:
John thought about baking a cake, but figured that it was moot because he didn't have the time to finish.
Solution 2:
Fruitless — M-W
adjective Producing no good results : not successful
"They made a fruitless attempt to find a solution."
"It would be fruitless to continue."