On my 20s, i know that is gonna prevent the deformity if i use the brace for my postural kyphosis, but is it gonna cause any bad issues for my spine?

Will likely not be: Helpful at all. You are done growing at age 20, so there is no reason to have a brace for this diagnosis. You have to see a physical therapist ; learn some very effective trunk stabilization programs, core strengthening programs, postural educations. The brace will not change your deformity, ; will only likely cause pressure points ; inconvenience. See a spine surgeon if you are having symptoms.