Custom MEP part not connecting

[Achille Pavlidis]

I have created a custom MEP part with MEP connections objects on the sides.

But when I load it on the project and try to connect the various ducts, Archicad refuses to connect. A forbidden symbol appears. 

See attached image

Why is this happening?

Operating system used: Mac Apple Silicon 13 Ventura

Edit: I found the solution, I opened the library part, and changed the MEP system from "Undefined" to one of the project. I think this should be added in the instructions