The bootloader must be unlocked before you can flash any custom software. For the P40 Pro (Kirin 990 5G), official codes are unavailable, leaving only two main paths: 10 Best Custom ROMs for Android You Can Install - Beebom

The P40 Pro hardware is excellent, but the custom ROM scene is dead due to Huawei’s anti-tamper measures.

on the official firmware is the safest and most functional route. sideloading guides for Google apps on your current EMUI version?

The Paradox of Liberty: Custom ROMs on the Huawei P40 Pro The quest for a custom ROM on the Huawei P40 Pro is a journey marked more by historical barriers than by technical triumphs. While the device remains a marvel of 2020 flagship engineering—boasting the Kirin 990 5G chipset and a legendary Leica camera system—it stands as a fortress against the very "Android freedom" that custom ROM enthusiasts cherish. The Unbreakable Gate: Bootloader Restrictions