Is this a rack model or desktop? (Model number / generation and similar specs would be helpful.)
Most Proliant machines enter BIOS through either F10 or F11 being hit constantly once you hear the initial "beep."
Plug in your USB boot stick, power up and go into BIOS. This takes a minute; be patient!
"Advanced" tab should give you some insight as to whether or not the machine is in SATA/IDE, AHCI or RAID mode.
"Boot" tab should show you whether or not UEFI is being enforced, what the boot priorities are (you'll want to set this to your USB stick), and depending on the model's age, whether or not UE can be enabled/disabled. If so, DISABLE UEFI by setting everything here to LEGACY or LEGACY ONLY modes, if the boot menu is showing as "Advanced Boot Menu" hit that and see if it's replaced with "Normal Boot Menu" and be sure NETWORK and OPTICAL boot options are removed for now (highlight the item and press "x") leaving just USB and HARD DRIVE on the list. You should also have a menu "Drive Order" where you can set the priorities of your devices. Your boot stick has to be plugged in prior to going into BIOS otherwise it will not be listed or even be an option.
* Leave room for discrepancies - this is all by faded memory as my two Proliant rack servers are either in the warehouse or basement awaiting their walk on the Green Mile.
If you're trying to install Linux on the machine because it failed when trying to install Windows, please note this as well.
If it's a desktop model, REMOVE the plug-in RAID controller before you do anything.
If it's a rack model... it's a "set" of challenges, thus why a model number would be prudent.
Good luck!