You need to unlock your bootloader.
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Enable USB debugging in the developer options;
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Reboot the phone in fastboot;
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Connect the phone to your PC;
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Run the commands bellow, line by line:
touch key.txt
fastboot oem get_unlock_key key.txt
fastboot reboot-bootloader
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It should show a warning that the bootloader is unlocked;
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Reboot in fastboot and install TWRP again.
Pixelfed is more similar to instagram and Mastodon to X (Twitter). They are different ways to access the Fediverse, Lemmy and Kbin are similar in that way (though Kbin can also use a interface similar to X).
And posts in Mastodon and Pixelfed can be seen by each other’s users as long as the respective servers are federated. So choose your preferred app/interface and post freely. From what I’ve seen Pixelfed is used mostly by photographers.