Macook Air Prohibited Icon booting from USB

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.

What model do you have? To find the model identifier open System Profiler in the Utilities' folder. It's displayed in the panel on the right. You may also find the complete model information by selecting About This Mac from the Apple menu.

If you wish to download from Mojave, then you need to erase the drive first. Why do you want to downgrade from Mojave?

There is only one way to downgrade. Erase the drive and install Yosemite from a full installer or a bootable installer on a USB flash drive. If you have a solid Ethernet or Wi-Fi connection, then you can do this:

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, at or before the chime/logo, 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 target drive (will be the out-dented entry) from the side list.
5. Click on the Erase button in Disk Utility's toolbar. 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 Install OS X and click on the Continue button.

This should install the original version of macOS installed when it was new.