Hi,
the error states that for the number range object REQREQ the interval with the ID PO is not defined.
You either need to rework the first step in you sequence:
IMG: SAP Implementation Guide > SAP Supplier Relationship Management > SRM Server > Cross-Application Basic Settings > Number Ranges > SRM Server Number Ranges > Define Number Ranges for Shopping Carts and Follow-on Document
Instead of using number 73-77 use the ID PO for the range that corresponds to your backend PO number range.
Or you need to rework step 3
IMG: SAP Implementation Guide > SAP Supplier Relationship Management > SRM Server > Cross-Application Basic SettingsDefine Number Ranges per Backend System for Follow-on Document
In this case assign the correct number range for PO's to you backend system --> NR_RANGE_PO = 75
The standard checks if he finds an entry in this table. If not, then he automatically searches for the number range with the ID PO which in your case doesn't exist.
Hope this helps
Markus