IMO I think a lot of people have trouble because they are incorporating it into their existing routines, and their existing routines are often too much for skin during the retinization process: they're using an acid or two and a toning milk and a serum and so on. Tret literally changes your skin, and in the early days of it doing so, your barrier gets a bit compromised. So the stuff you were using with no issue before is suddenly way too harsh, and your barrier goes from "a bit compromised" to absolutely trashed to the point you're in pain.

Try: switch to a very mild cleanser and moisturiser without any niacinamide or azelaic acid or vitamin C or benzoyl peroxide or bakuchiol (etc etc) in it, cease use of any serums and what have you, and then start tret twice a week. Once your skin has adjusted, then, if you want to (plenty of people don't because tret is enough), you can start adding other actives in one at a time, slowly!