Because that would be "How much rice is there?"

Exactly. Mennyi a rizs?

It literally means "How much is the rice?" (including both meanings). However, this is not the same as either...

• Mennyi ez a rizs? = "How much is this rice?"
• Mennyi rizs ez? = "How much rice is this?"

It's about whether you want 'rice' or 'this' to be the subject

In the Duolingo sentence, it's clearly 'it,' so you must not let 'rizs' become your new subject.