In my experience that flashing underscore is symptomatic of the BIOS being unable to properly boot the device you've told it to boot (the USB stick, in this case).
On 10 series like ours, you should be able to boot a properly built USB stick with minimal BIOS edits. Like, load optimized defaults, and I can't even think of a setting you need to change for Clover to load (macOS is a slightly different story but I make very few changes). AHCI mode for your SATA devices should be the default, and the legacy/UEFI thing is fine to leave at the default for now. Even XHCI and EHCI are about USB.
So that tells me it's either likely you made too many BIOS changes and locked yourself out somehow? or the stick hasn't been built correctly. Did you verify that the install macOS High Sierra app that you downloaded, and that Unibeast used to build the USB stick, is the 5GB or whatever in size, and not 19MB, as some folks report? You used the latest Unibeast, and created the stick in UEFI mode according to the guide? You're using a USB 2.0 port on the back (upper back) part of the machine? Sticks can be wonky, you could always try rebuilding the stick using another stick you have around. And when you hit F12, you select the USB stick that does NOT have a P (e.g., P0, P2) in front of it? The P# in front of a drive means boot in legacy mode, not UEFI mode, and you built the stick in UEFI mode so you need to boot it in UEFI mode.
