In AX 2012, you can specify which items can be purchased from what vendors. This prevents the purchasing/buyers departments from purchasing items from a variety of vendors at different prices and qualities. This gets especially important when it comes to time sensitive purchasing contracts from specific vendors.
If you are getting the error, it is because the vendor on the purchase order is not setup as an approved vendor to purchase from on this item and the item is configured to throw a warning or error.
Approved vendor list setting
You can setup approved vendors by navigating to the Released product. Click on the 'Purchase' tab in the navigation ribbon and click 'Setup' under the 'Approved vendor' group (Figure 2 below). Here you can specify the approved vendors with valid to/from date. Note: this vendor approval is at the item level, not variant/inventory dimension level.
The other two options in the 'Approved vendor' group are view only any allows you to see all the approved vendors for the item as well as the effective periods for a given date. Its really just two filter for viewing approved vendors. This could have been one form in my opinion but I can see why they did it.
You can also view all the items that are approved for a given vendor by navigating to the vendor and clicking on 'Approved vendor list by vendor' option under the 'Related information' group in the 'Procurement' ribbon tab (Figure 3 below).
Vendor Approval setting on the item
On an item by item basis (e.g. setting on Released Products), we can specify if we want to ignore the approved vendor lists functionality ('No check'), only throw an error if the vendor should not purchase from the order ('Warning only'), or halt the entire process ('Not allowed'). These options can be seen in Figure 4 below.