Huawei officially stopped providing bootloader unlock codes in May 2018 . Since the Y9 Prime 2019 was released after this date, there is no official/free method from Huawei. The methods below require paid services or third-party tools.
The is now several years old. Unlocking the bootloader is no longer about keeping up with security patches (Huawei stopped updates for this model in 2022). Instead, it is about: unlock bootloader huawei y9 prime 2019 verified
After unlocking, do NOT relock the bootloader unless you have a full stock firmware. Relocking with modified software will brick your phone permanently. There is no official TWRP for STK-LX1, but