Is it normal to have prohibitory symbol on brand new Mac Air ?

Prohibitory symbol

A circle with a line or slash through it means that the selected startup disk contains a Mac operating system, but it's not a macOS that your Mac can use. You should reinstall macOS on that disk.

Get MacOS Mojave now from the Mac App Store. Afterward, you can install the update, Download macOS Mojave 10.14.1 Update.

Or, use Internet Recovery:

Internet/Network Recovery of El Capitan or Later on a Clean Disk

If possible back up your files before proceeding.

1. Restart the computer. Immediately after the chime hold down the (Command-Option-Shift-R) keys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the drive (usually, the out-dented entry) from the side list.
5. Click on the Erase tab in Disk Utility's main window. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
8. Click on the Apply button, then click on the Done button when it activates.
9. Quit Disk Utility and return to the Utility Menu.
10. Select Reinstall OS X and click on the Continue button.

Note:

1. To install the version of OS X that was currently installed use Command-Option-R.

2. To install the original factory version when the computer was new use Command-Option-Shift-R.

If you are experiencing an issue not listed here, contact supportContact support for issues not listed here.