Since you didn’t show your BPI, I assume you have used a blueprint reference in your interface function as input to access the other blueprint. It is the same as casting. Just remove that input.
Now, if you have a BPI function that has no inputs and outputs, you are to add the interface to the target blueprint (the blueprint to which you want to access). And in that target blueprint, implement the interface. All the nodes that have that target blueprint’s variables, functions, events, macros etc. go there. Then, in your source blueprint, send a message to that BPI function after the nodes that belongs to your source blueprint. Then, the rest of the code in your source blueprint go after it.
To send a message to your blueprints, if it is the character blueprint you want to access, use the Get Player Character node as target. If your target blueprint is the controller, then use the Get Player Controller node instead.