Please SIGN THE PETITION!
Microsoft is moving all Mojang accounts to Microsoft accounts. This breaks Minecraft Java on RPi, but a fix is in the works and will take a while to be released.
Sign the petition: https://feedback.minecraft.net/hc/en-us/community/posts/360075486052-Port-the-latest-Java-Minecraft-Launcher-to-the-Raspberry-Pi

NOTE: For best performance, we recommend a version like 1.12.2 + Optifine.

Minecraft Java edition for the raspberry pi
To maximize performance and to get solid 60fps, please follow ETA Prime's excellent walkthrough video: https://www.youtube.com/watch?v=oqNWJ52DLes
To run: Menu -> Games -> Minecraft
To run in a terminal: java -jar /usr/share/minecraftjava/launcher.jar
This installs the old Minecraft launcher so that MC can be launched properly with any version and Optifine.

Botspot has configured the install script to create a custom launcher_profiles.json so you don't have to change settings manually.