Hi!
That numbering object RV_BELEG, belongs to the number range assigned to sales documents. In this case, the number range that is assigned to your billing document does not have a valid number range created, or your billing document does not have any number range assigned at all.
In the customizing of your billing document (trx VOFA), you can see the number range ID that is assigned to it:
You can create the number range you need whether by using transaction SNRO, or through the following customizing path in SPRO: Sales and Distribution --> Billing --> Billing Documents --> Define Number Range For Billing Documents
Regards,
Andrea