What causes this issue?

Factory calendar &V1& is not recognized.

How to fix this error?

The error message has two possible causes:
Check the entries in the customer calendar for the unloading point on
the unloading point screen in master record for the ship-to party.
Check the billing date entry in the billing data in the customer master
record and in the billing details data in the header of the underlying
sales order.
Note the special rules for billing dates defined in the factory
calendar, if relevant.

Error message extract from SAP system. Copyright SAP SE.