per wiki:
Loading with debit card at Walmart
Self checkout does not work for loading.
This is true for the SCO registers but there is also a regular register there that you can request a cashier/attendant to make loads for BB, Serve or even pay WM CC. Just try it when they're not busy assisting other customers and they'd gladly help you
Would've added info to wiki but I don't know how to edit wiki.