I am trying to add a supplier part. I can see it in the dropdown but when I select either I get the error. It’s in the price list and approved suppliers list. Any ideas?
is it in the Supplier price list linked to one of your parts?
Yup 
the PO was created today? Effective date on the SPL looks like today.
Entering in your part 8000008 into your PO line the supplier part should populate automatically. Did you try deleting the line and readding?
There are two tables and vendpart and parrtxrefvend
partxrefvend is the one that is referenced in the supplier parts tab and is also used in the approved suppliers (parts)screen. The field name is VendPartNum
VendPart also has a vendor part num field VenPartNum. which appears to be empty. I have seen situations where people have used DMT for the supplier price lists and mistaken the VendPart.VenPartNum instead of using the PartxRefVend#VendPartNum.
Perhaps another thought have you selected the right supplier in the PO?