If you and your instructor agree then go for it! There are so much fun to regularly do and they teach you and your horse so much so honestly when you feel that you and your horse are doing the basics consistently well then try and get out to a small dressage training show. Even a show that isn't the discipline you train or want to compete in regularly will give you so much confidence with the next one and will still be super beneficial
work with your trainer, tell them you want to start going to shows see what classes you would take part in. its really your trainer and yours decision to know when you are ready. hope this helps!!
You’re instructor will tell you