Serial Number Unavailable

Hi ShashBin,

Welcome to Apple Support Communities. There are a few different places you can find the serial number since it does not appear in About This Mac. These are mentioned in Find your Mac model name and serial number - Apple Support. A couple of these options are as follows.

"System Information

1. Press and hold the Option key on your keyboard while choosing Apple menu  > System Information from the menu bar.
2. Select Hardware in the sidebar.
3. Find the model name and serial number on the right.

appleid.apple.com

1. Sign in to appleid.apple.com.
2. Click Devices in the sidebar.
3. If your Mac is listed on the right, click its name to see its model name and serial number."

You might also try safe mode to see if the serial number will show up there. To access safe mode, use these steps from How to use safe mode on your Mac - Apple Support.

"Intel processor

1. Turn on or restart your Mac, then immediately press and hold the Shift key as your Mac starts up.
2. Release the key when you see the login window, then log in to your Mac. 
3. You might be asked to log in again. On either the first or second login window, you should see ”Safe Boot” in the upper-right corner of the window."

Take care.