I payed fort Minecraft Java edition and I can only open the demo. I've uninstalled and then reinstalled Minecraft and launched it through Minecraft Launcher, again, took me to the demo version.
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We're tracking this issue in MCL-16570, so this ticket is being resolved and linked as a duplicate.
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i have but one thing to say... make a Mojang account and sign in. then open minecraft launcher and sign in with your newly made account