The ZenFone 5Z received Android 10 (Q), but if you participated in the beta program and tried to roll back to Android 9 (Pie) using a simple ZIP, you likely triggered a anti-rollback mechanism. Your device might show "No command" or boot directly to fastboot. Raw firmware is the only fix.
Sometimes a new update introduces bugs or removes features. OTA updates often do not allow you to downgrade (go back to an older Android version). Raw firmware allows you to wipe the current OS and flash an older, stable version (e.g., going from Android 10 back to Android 9). asus zenfone 5z -zs620kl raw firmware-