How to Make an Internal Outfall into an External Outfall when you have more than one link connected to an Outfall in SWMM5
Step 1: Identify the Problem "ERROR 141: Outfall J-561 has more than 1 inlet link or an outlet link." Means you have an outfall node in the middle of your model.
Step 2: Make a new outfall.
Step 3: Make the new Outfall have the same invert as the old outfall