"Roupa" in Portuguese can be either countable and uncountable.
You can use "roupa" as well as "roupas" in the sense of "clothing/clothes". ("Roupas" is also right and the system has been updated)
A similar thing happen with pairs. Often you will hear "meu sapato" meaning "my (pair of) shoes".
"They need garment" is not something we would say in English. "They need clothes" is much better. "They need garmentS" is acceptable, but it is not not very common to use garments. It sounds very old-fashioned. Maybe they still use garments in the UK, but it would still have an s on the end. By the way... I love learning with Duolingo!