Learn Italian: Questo è il dazio, non può discutere. - This is the duty, you cannot argue.

No. Dazio specifically refers to a financial duty, tariff, or toll, such as a customs tax. If you want to talk about a moral duty or a responsibility, you should use il dovere instead.

The English translation says 'you cannot argue', but the verb is può (he/she can). Why?

This sentence uses the formal 'you' (Lei). Because a customs officer or official is likely speaking to a traveler or citizen, they use the formal third-person può rather than the informal second-person puoi.

Does discutere mean 'to discuss' or 'to argue'?

It can mean both, but it often implies a much stronger disagreement than the English word 'discuss'. In this context, non può discutere translates best to 'you cannot argue' or 'it is not up for debate'.

This lesson is about using andare to express that something must be done. How would I use that to say 'The duty must be paid'?

You would say Il dazio va pagato. This pairs the verb andare (va) with the past participle (pagato) to express a strict obligation, which is exactly how the passivo di necessità works.