Hi, whilst your snap is marked as private it is uploaded to the global snap store and so needs to be treated as a publicly accessible snap (since you could easily mark it as public in the future).
So then looking at your snap - I notice your snap declares a large number of serial-port slots - however application snaps are not allowed to declare this since they don’t actually provide the serial-port at all - instead only gadget snaps can declare a slot of this type.
Instead I suspect you want your snap to be able to interact with whatever serial-ports are connected - in that case you likely want to use serial-port via hotplug - please see Hotplug support in that case.
Otherwise, please provide more details on what your snap is trying to accomplish by slotting serial-port currently and we can try to help you achieve this in a more correct manner.