Hi,
How a Raw Material is created with valuation class?
Use t.code:MM02, enter valuation class in accounting-1 view and save,If valuation class allowed to change for material with field selection for your material type{check control on Field Reference (OMS9) and Selection Group (OMSR), also check your needed valuation class linked to account category reference}.
When a material do not have valuation class and you are creating purchasing document, system should have asked you to enter account assignment category- so you created your purchasing document.
Now before changing valuation class, set deletion flag to the item and then change valuation class, as your material is stock material and create purchasing document.
Regards,
Biju K