This can happen if USB ports are disabled on BIOS. Did you check USB ports on front and rear of the chassis. If you have access to iDRAC you can check BIOS setting to see whether USB ports are disabled. [URL] Another option is to reset BIOS to default using NVRAM_CLR jumper and check the behavior. Refer below link for details [URL] 
I recently inherited a Poweredge T620 from my stepdad who passed away. I finally got around to hooking it up to check it out and I'm running into an issue where there I believe is a bad memory module (A4) but I can't get a USB keyboard to work in order to run more diagnostics or even get it past the screen, I'm stuck at Press f1 to continue, or press f2 to run the system setup program. I'm not well versed in Server hardware and could use some guidance or suggestions. 
Power Edge t620 USB not recognizing