Number range interval & of object & is not internal

What causes this issue?

The number range interval &1 for number range object &2 is not
internal.

System Response

Processing cannot continue as the program must get an internal number
from the number range interval &1 with the function module
NUMBER_GET_NEXT.
The program is canceled.

How to fix this error?

Call the transaction &3 to maintain the number range (or Transaction
SNRO, Object &2, Go to -> Number ranges).
Delete the external number range interval &1.
It will then be created consistently as an internal interval at the
next call, and can be used to get the required internal numbers.

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.