Cannot assign VAT registration number to a customer that is Legal entity in the database

Hello

we are in r12.1.3.

I have 3 Legal entities and 3 ledgers and 3 Operating units for the same country.

I want to create a customer in one of the Legal entity.

The customer is one of the other Legal entities.

I am able to create  the customer header, the customer site (ship to, bill to, legal to).

But I got an error when assigning the VAT registration number at Account level in Tax Profile tab.

I got the following error:

"This tax registration number is already in use for party type: Third Party Site and party name: for the same period."