Failed -remote 39-flashing Lock Flag Is Locked. Please Unlock It First 39-- ((better)) < 100% PREMIUM >

(a custom ROM, a factory image, or a recovery like TWRP)?

Avoid using USB 3.0/3.1 ports (often blue or red). Fastboot is notorious for failing on newer ports; use a USB 2.0 port or a USB hub if possible. (a custom ROM, a factory image, or a recovery like TWRP)

For other platforms: