In my current project we had to remove the Global Address Book Solution for Dual Write from the Environment. During this process, the following error was occurring on Customer Create.
Write failed for entity CDS Contacts V2 with unknown exception - Field 'Associated party number' must be filled in.\nvalidateField failed on field 'smmContactPersonCDSV2Entity.AssociatedPartyNumber'
The problem was that the plugin "Microsoft.Dynamics.SCMExtended.Plugins.Plugins.LeadPrimaryContactPostCreate: QualifyLead of lead" was still deactivated. This Plugin is about to be disabled on install of GAB (Link) and this Error is what you are facing if you do not enable it afterwards.
Cheers
