With your bootable flash drive attached, press F12 a few times at powerup.  Choose the flash drive from the boot menu.

Do you have a bootable USB? If you do, 

1.insert the USB,  launch the download file, choose “Create installation media", finish the language and edition settings, then choose "USB flash drive”.

2. Restart your computer, and press a required key as the screen shows. And follow the settings in the window.

3. "Install now", and activate windows with your product key.

4. “Custom: Install Windows only(advanced)” and select the drive you need to install Windows on. 

Then your computer will restart several times, but it's normal, just wait for it.

If you don't have a bootable USB, you need to build a bootable USB at first, you can download Windows 10 ISO files from Windows official web, btw, the USB should at least have 5GB free space. You can also create Win10 USB via other software, it may be easier. The following steps are to make a bootable USB with CMD.

1. Open the CMD window, type diskpart, hit Enter.

2.Type these orders one by one:

list disk

select disk x(the USB)

clean

create partition primary

select partition 1

format fs=ntfs quick(If you need to create a UEFI bootable USB, you should type format fs=fat32 quick)

And then, the install steps are as same as the 4 steps mentioned above.

The flash drive is not offered as an option. It only offers me to boot from the HDD/SSD/(stick)

Can't launch any files if the machine won't boot from USB.

Yup. Multiple times.

This might be solved by now but for anyone else who might need this info, have a look at Rufus or windows media creation tool for Windows 10 and Windows 11.