There are people who cannot sing but they are rare. I've only met about three such in my life. I'm yet another who left school believing they couldn't sing. I was very lucky in that I got involved in the folk scene in the early 70s, a time when there was a huge amount of egalitarianism when it came to singing and a lot of encouragement to do so - a characteristic that is still present to a significant degree in folk clubs even today. If people expect you to sing, well, you end up singing.
Having said that, once you've started singing you still need to improve and for that you might need help. Singing teachers can be good. I was lucky in that I came under the influence of one Frankie Armstrong while she was still developing her ideas. At that time she taught a style of singing derived from the Balkans that basically involved giving it full on welly. I cannot recommend that too highly. If you can sing with some power you can always wind it back, but you can also belt it out when you need to. Singing, say, a sea shanty with a full-throated holler is a lot of fun.