Reboot your device into recovery mode by pressing and holding the Volume Down, Power, and Bixby buttons simultaneously.
The Samsung A9 2018 is not dead for custom ROMs, but the Snapdragon 660 + Samsung proprietary camera stack limits it to a mid-tier custom ROM experience . Use LineageOS 20 unofficial if you prioritize latest Android and can sacrifice 2 cameras + VoLTE. custom rom for samsung a9 2018