Exactly the same problem here, just crafted a nekomata and can't attach a scope. Same with attatching a mod to the black unicorn

From what I can tell, sniper rifles use a different set of scopes from the other weapons. Also I think - but I'm not a hundred percent sure - that precision rifles (not sniper rifles) don't have access to all regular scopes either, though it should be most of them.

What bugs me is that there is a distinct lack of information for mods and weapons: It doesn't say which weapon types a scope/mod can be used for, and as far as I can tell, there is no level indicator for weapons, either (which would be really useful when attempting to upgrade weapons, i.e. so I know how many more times I can upgrade a given piece of equipment at my current level).

EDIT: Scratch what I said about precision rifles. Checked it once in game and they can use all regular optics like the other weapons.