No appropriate external grouping exists for partner number &1

What causes this issue?

You attempted to use an external business partner number that has no
external number range interval.

System Response

The creation of the business partner is terminated.

How to fix this error?

Enter an external number that has an appropriate number range interval.
If you have to use external numbers, check if it is possible to change
the number range intervals. You can check the groupings in transaction
BUC2 and the assigned number range intervals in transaction SNRO (number
range interval BU_PARTNER). Any changes must first be agreed on with the
system administrator.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.