Internal number assignment. Enter the external account number

What causes this issue?

You attempted to enter one of your own account numbers where internal
number assignment is used.

System Response

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

How to fix this error?

The type of number assignment is set in this case in the number range
for accounts (Customizing).
If you have set this to external number assignment, you will have to
enter the account numbers yourself.
For internal number assignment, you enter the external account number.
When the account is created, a unique internal number will be assigned.

Error message extract from SAP system. Copyright SAP SE.