I had the same thing on OP6T, rocket2infinity answered in Telegram, Welcome and Install page on Sep 13th, but here it is in a nutshell : "No need to install it. Just boot into TWRP after unlocking bootloader and setting USB permissions under developer settings in Android. Then wipe cache and data. Reboot to fastboot, then run installer without rebooting back to Android"
It worked for me.