Error message :Grouping has external number assignment. Please enter a valid number.

[SAPSupport]

Error message :'Grouping has external number assignment. Please enter a valid number. ' While creating the Business Partner.

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[SAPSupport · Accepted solution]

Please check the setting in the following configuration setting:

1. Open the Manage Your Solution app.
2. Open the configuration step Define Groupings and Assign Number Ranges (SSCUI 102241)
3. Change the Number Range to an internal number range and the business partner will be created with the next available number

OR 

Enter the next available external number.

For further information see Business Partner Number Grouping