Diagnosis
This number range does not exist in the number range file for this object. The internal number assignment cannot therefore be carried out.
&WHAT TO DO&
The number ranges for the different objects can be maintained via Customizing. The number range for the object "Partner" can only be assigned with indicator "AP".
Maintain number range for the object "Customer"
Maintain number range for the object "Vendor
SOLUTION:
STEP 1: Go to T code XDN1 and click on Interval

STEP 2: Maintain the missing number range A1 by providing number range Interval and Save.

Thanks,
Adharsh K S