If you've read the recent MC account migration (https://www.minecraft.net/en-us/article/java-edition-moving-house) and you use Technic Launcher (or any other custom launchers), then do not migrate your account (to a Microsoft account) or you will be unable to use the launcher.
The new auth system that Minecraft uses after this migration isn't support by any MC launchers other than the vanilla launcher, nor do we currently have any information about it so we can implement it.
When? According to Mojang, you'll be forced to migrate your account starting in early 2021. We hope to have support for their new auth system by then. The migration is currently not ongoing.
What if I create a new Minecraft account? Minecraft accounts created on December 1st, 2020 and later, are bound to Microsoft accounts already, and won't be able to use the Technic Launcher, or any other third-party launcher, until Mojang/Microsoft create a solution we can use.