The existing billing documents cannot be invoiced (CA &1)

What causes this issue?

The invoicing unit for the selected account &v1& could not be created,
because the internal validations could not find any invoicable billing
documents (automatic or manual).
This could be due to one of the following reasons:
Only a manual billing document exists for the contract account to
invoice, and it may not be invoiced by itself. No leading automatic
billing document exists.
More than one required contract exists for the contract account to
invoice. However, not all the periodic billing, interim billing, or
final billing documents are available for all the required contracts.

System Response

The contract account was not invoiced.

How to fix this error?

Check the billing documents for the contract account that have not been
posted.
If no leading automatic billing document exists, make sure that the
existing manual billing document is not invoiced until at least one
additional, automatic billing document exists for the corresponding
contract account.
If more than one required contract exists for the corresponding
contract account, make sure that at least one automatic billing
document is available for all of the required contracts that belong to
the corresponding contract account.

Error message extract from SAP system. Copyright SAP SE.