Instead of doing it in a BPM can you do it in code on the customization instead? You have to get your fields from either a grid you embed or a dynamic query, but if you can get that you can call the BO from the button click event.
It’s similar to this thread where I learned how to call LaborUpdate in code.
The other option would be to build the BPM on a UBAQ and call that BAQ with your button. Depending on what you are doing that could be easier or harder.
Last edit: on the error, maybe you just need to change your RowMod to U instead of A?