Yes, it's wrong, it's "O prazo".
But....if the translation was: "they will lose in the long term", then "a prazo" would be right (preposition + noun).
We use "a prazo" very commonly when buying stuff. There are two buying options: "à vista" or "a prazo".
- À vista = single payment
- A prazo = the price is divided to be paid monthly (in the long term)