Obviously, your software (like all pretty much) runs iterations trying to converge to balance flows and pressures, but it seems to be failing. Your best option is to contact the support of the software company. The very least I could say is to check your design criteria and play with pipe sizes, maybe number of nozzles, system geometry etc. Often complex systems lead to unacceptable results that can be mitigated, up to an extent, by changing the design.