The word “assignable” is translated to the German »zuordenbar«, according to several dictionaries. I have never heard this word and it sounds wrong to me. Is it being used?
It's probably just as common as assignable ... that said, there's nothing wrong with it. Duden has it, so that should settle it.
It sounds very strange to me (German native speaker), I've never heard it before and I would think it's a spelling mistake.
I would use zuordbar (without en) instead.
It's derived from zuordnen, which means to assign or to map or to pair, or something like that. As in "Assign each of these cities to their respective states".
As another_wald has pointed out, in a programming language, or when you talk about assigning tasks, you would probably use zuweisbar" instead, which makes clear that a new assignment is being made ("At his first day at work, he was assigned the most boring task"), as opposed to just putting things together, like cities and states. But zuordbar can be used for that as well, whereas zuweisen always creates a new assignment.