Standard OnePlus devices can often be unlocked with a simple fastboot oem unlock command. However, carrier-branded variants (like the OnePlus 8 or 9 series from T-Mobile) have an additional security layer. The bootloader is restricted until a specific provided by OnePlus is flashed to the device. How to Fix the Error
: Use the volume keys to select "Unlock the Bootloader" and press Power to confirm. Why this is "Better" please flash unlock token first oneplus better