i cannot play with my physical copy of The Witcher 3: Wild Hunt. I bought it in 2015 and register with my old account on gog. Now i have a new account, my old mail and old gog account didin't exist anymore, but i cannot register my game with my new account. I paid for it and i don't understand why cannot play. What can i do to play with a game i paid for?

You used your key on your old account. You can only use it once. That key is tied to your old account.
GOG doesn't just remove accounts, so your old account should still exist. If your old e-mail doesn't and you ave forgotten your password, that's a problem.

You might try contacting support, but I assume they want some form of proof. The box itself is no proof. Someone might have given or sold it to you.
If you still have the receipt GOG send to your old acocunt, you might add that to your support ticket.