How can I turn off the Power on sound of my Android phone. When it doesn't have power off sound setting?

There's no setting for it. it was designed to make sound Everytime You turn on your phone on. all you have to do is to cover the speaker of your phone so that Annoying sound will never be heard by your subconscious ear maybe.

There should be a setting to disable it in sound and notification settings. From your settings menu go to sound and vibration, if there is no option on the page to disable it, look to see if there is a "more sound settings" option , if there is, open this and the option to disable the startup sound should be there, just move the slider to the left and you should be good to go!  This is the procedure to disable the startup sound on my Huawei  p30 pro

Disadvantages of Android phones. Need more Upgrade!

The Cellphone masters knows How to silent your phone Power on even there's no settings for it.

Hi. 666 my phone too don't have settings for start up sound. This must be very rare.

From the home page go to Apps then tap on settings. scroll up look for Sound and tap on it. Now scroll up and turn off the button next to "power up sound". Now when you restart your phone the start up sound will be muted.

---How to stop the start-up sound on Android Nougat 7.0 phones

The phone I brought from local also cannot be turned off the Power On sound.

You can just cover the speaker of your phone to mute the "Ugly Power On ringtone"