You can look out for the following issues, as there may any factor from them that's causing the issue:
1. Check if your USB drive is bootable. Before you enter into the BIOS to alter boot order settings, make sure that your newly prepared bootable USB drive is really bootable.
2. Check if your PC supports booting from USB. As you likely know, one can’t boot from a USB drive on a machine that doesn’t support booting from USB. In order to check if your PC’s motherboard support USB booting, you can either check your PC’s manual or enter the BIOS to see if USB boot option is present.
3. Configure your PC to boot from USB. If your USB drive is bootable and your PC supports booting from USB, then you need to walk into the BIOS and configure it to boot from USB. In order to enter the BIOS, you need to reboot your PC and press a specific key as soon as you see your PC reboots. Most OEMs use either Delete, F2 or F12 key to enter into BIOS but we advise you check your PC’s manual or PC manufacturer website for the right key to enter the BIOS and to know how to configure your PC to boot from USB.
4. Check the file system of your USB drive
If your PC supports UEFI and you’re trying to install Windows in UEFI mode, you should make sure that the bootable USB drive is formatted in FAT32 file system.
Hope it helps.