Reboot to bootloader and reboot to fastboot. Next boot TWRP img file through adb fastboot tool as command: fastboot boot (drag img) [press enter]..
TWRP : https://androidfilehost.com/?w=files&flid=326293
Soon you'll be on TWRP recovery and do press the (advanced) button left below, reboot to EDL while you have already opened the MSM tool and have chosen O2 instead of Target above. As soon as the tool says COM is connected, press start and wait for a while. Your phone shall be new as it was. You need to unlock OEM again. Greetings
Get the file here: https://www.droidwin.com/unbrick-oneplus-9-pro-msm-download-tool/
Do choose the Global version if you want to be the first to get the updates.