Digital signature for number range number &1 is inactive in Customizing

What causes this issue?

You tried to post a document that requires a digital signature, but
digital signature generation is not activated in Customizing for the
corresponding number range interval.
Billing documents that are posted in a Portuguese company code or in a
Portuguese organizational unit of a foreign company code must be
digitally signed.

System Response

The system prevents the posting.

How to fix this error?

Procedure for System Administrators

You must activate digital signature generation for this number range
number if one of the following applies:
No digital signatures have been generated for this number range number
yet.
Digital signature generation has been activated for this number range
number and then deactivated, but no documents have been posted for this
number range interval while digital signature generation was
deactivated.
<ZH>In all other cases</>, you must create a new number range number
with digital signature generation switched on. To avoid inconsistencies
in the database, you must use a different combination of leading
document type and series.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.