What is the route?

You can only use M+C on DL operated airlines. If the ticket in question includes partner metal, that's the reason you are error-ing out and not getting a true miles + cash option.

If on DL metal you should be presented with up to three options and then one bonus option if you have an AMEX:

1) Cash
2) Miles
3) Miles and Cash

Which you can switch between by selecting the category at the top of the page.

The fourth option is Pay With Miles where you have to first select the cash fare and then if you have an eligible DL AMEX, you can offset the cash price with your miles.

If you are flying on partner metal for any part of your trip, your only options are cash and miles, there is no M+C option and no PWM option