I'm sure this has been answered already somewhere, but i didnt see it on the first 6 pages.

Mishima Zaibatsu Leader :

E Commit: Your opponent discards one of their momentum. Then you add one card from the top of your deck to your momentum.

Does your opponent have to have momentum for you to be able to use this effect to get momentum?

Yup. Key word is "then".

"then" means only do the 2nd part if the first part successfully finished. So if your opponent cant discard 1 momentum (iether cause they have none or cause of an effect not allowing momentum to be discarded) then you don't get 1

It's a good card. Without the restriction it would not be a 5 check, have a block, and probably be higher difficulty to balance it.

"free" momentum is hard to come by in this format, usually hinges on doing dmg (if this attack deals dmg, add top card), etc