A list of the complete business rules (BR) and business terms (BT) can be found in the official EU norm EN 16931 (German).
The list of error messages is constantly being expanded.
Value of '@unitCode' is not allowed
The unit codes that can be used in e-invoices are standardized by certain codes. An invalid code has been used here. Possibly a typo during development.
Solution
A valid code must be used. The code lists are part of the EN 16931 (German) or ZUGFeRD documentation.