Custom Recovery blocked as a security measure?

Hi Everyone,

I originally flashed LineageOS and it wirked beautifully. Then I thought I wanted root.... After trying some things that didn't work I flashed my tablet back to stock to get it working again and then attempted to reinstall LineageOS cleanly. But now, after flashing the recovery with Heimdall, when I power off with Vol-down and Power and immediately switch back to Vol-up and power, the regular warning screen shows up with the standard message: "This tablet's bootloader is...

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