@illspiritx is exactly right, and that brief answer is 100% correct.
To add on top of that: What you’re adding is not a “variable” but an “output value;” when I clicked into this thread I expected a different question. Something to keep in mind for the next question, to reduce confusion.
Also: To return values in interface functions, you need to implement the interface with a function, rather than an event. In general, functions are the “cleaner” way to organize Unreal classes / blueprints, but I understand the convenience of events. (As usual, the convenient thing is only the right thing when the task is small and well contained …)