I can't thank you enough!
I deleted all field and only had number and displayName, this allows the POST to work with no issues.
Leaving the number field blank and only populating the displayName with my current No. Series configuration returns the following error:
"error": {
"code": "Application_DialogException",
"message": "It is not possible to assign numbers automatically. If you want the program to assign numbers automatically, please activate Default Nos. in No. Series ITEM. CorrelationId: c6ad8048-7d37-4cf8-9182-bbf162213cce."
}
But this is perfect, I can continue now.