You may have the whole drive partitioned for the running (already installed) Linux OS
--the usb may not be able to boot correctly unless discovered on the boot sequence
If you have some free space on the hard drive (an allocated, free or unformatted partition), then you may be able to install if the usb can be booted from another OS
--it would also help to know which product you used to create a bootable usb (is it bootable or not
); you can test if it is by assigning the usb device as the boot device in the BIOS of the machine and doing a test boot
If that doesn't work, then your usb is not a bootable device ??