Yes, it is an error. "Potrzebować" requires genitve and it applies to noun and all that is associated with the noun - adjectives, numerals, pronouns etc.
- Potrzebuję sześciu litrów wody
- Potrzebuję dobrego wina [I need good wine]
- Potrzebuję jakiegoś samochodu [I need some car]
See also "Czego potrzebuje pies?"
Right, when someone else (or us) come across this in an exercise again we'll do that. The concept that even numbers change according to case was a hard one to swallow, so I made myself very sensitive to this :P
Some language speakers, while this is technically incorrect, would actually speak like that. I do too, actually, convert the „potrzebować” genitive to nominative (or accusative?) if there is quantifier included (numbers or otherwise).
While not very helpful at first, it's good knowing "common errors" made by native users to learn understanding casual language better. Just imagine how it would feel reading every incorrectly used "it's", "its", "they're", "their", etc. literally on the Internet.