I bought Minecraft and have my receipt for the game and when I try to sign in using the TWITCH app or MULTIMC or ANYTHING but straight into the game or website, it will not allow me to mod anything. EVERYTHING says I did not pay for the game- I have an account, but it is an unpaid account. I bought the game and can NOT find an answer to this issue ANYWHERE in SUPPORT or online... this forum is my last hope.
Contact Mojang, they are the only people who can help with account issues. Unfortunately, they are a few months behind in support tickets at the moment, so it's going to be a while before they respond to you.