TLDR: Make sure the USB drive is USB 2.0 and under 32GB and the HP USB Key Utility must be used.
Had a bit of a game with this today, server not updated since 2017 so with a couple of versions of iLO to upgrade through thought it would be easier to put the SPP on a USB drive.
Downloaded the ISO (which took forever to find, thanks HP) and imaged with Rufus which didn’t work. Then found the HP USB Key Utility and tried another 2 USB 3.0 drives which also didn’t work. The server recognised the drive but when trying to boot it wasn’t having it.
One 32GB USB 2.0 drive later and it’s finally booted.