Profinet Error (PNIO0025): Profinet '...' module is missing a submodule
There are Profinet modules that do not have "fixed" submodule (subslot) elements but can be equipped individually according to the application needs. According to the Profinet specification, the configuration is only valid if each module has at least one submodule.
PLCnext Engineer detected such a configurable Profinet module in your project which does not contain any submodule. This is not allowed.
Deleting the last submodule is not possible for Phoenix Contact Profinet modules. Therefore, the error has been reported for a third-party module.
Remedy
Add submodules to the Profinet modules or delete the "empty" module from the PLANT.