When a mix is setup it needs to be marked as a mix, have at least 1 formula set up and a default formula added to product. Missing one of those steps leads to posting errors. It would be nice if the Products module and Mixes module would display a message stating the steps needed (missing) when they save a record that is a mix.
Ford, the only required fields for a Product Mix are LocationID, ProductIO, and MixID. The formula is not required.
On the Product screen you could go into Screen Designer and make the Default Mix field a Required field so they would have to populate it before they could savce the record.