Can I Install Libreelec Without Usb And How

Two comments:

a) If you can't boot from a USB stick, you might need to pull the drive and connect it to another PC and run the install from USB there; or write the installer image directly to the HDD and then (in the old PC again) interrupt boot and type "run" (direct from install media, which is now the HDD) instead of following the normal "install to target" flow.

b) LE is small and lightweight which is appealing for older hardware, but we target current and recent HTPC hardware so are often missing the drivers and such needed for old hardware. If you have something so old that it won't boot from a USB stick, a more conventional distro that supports a wider range of hardware might be more appropriate.