I downloaded the demo version, then I bought the game, I downloaded the client I downloaded a zip minecraft server 1.12.2 (which I do not know where to put) and when I open the minecraft luncher tells me that I can only play the demo version. Because? where did I go wrong? (I have doubts about having bought it twice as well because months ago I bought the version for windows 10 but did not find any servers to add)
Can someone help me?
You're logging into the wrong account. In any case Contact Mojang, they are the ONLY people that can help with account issues.