I can't boot the USB installer! What's wrong?

Have you turned Secure Boot off?  TLXOS does not support Secure Boot.

There are some other known issues - we'd argue they are BIOS bugs - with certain UEFI BIOSes.  Known offenders include Dell Optiplex models, some older HP/Compaq laptops, Intel Compute Sticks, and certain NUC models (mainly the more recent ones, plus the notoriously buggy DE3815TYKHE).  Issues and possible workarounds may include:

• The BIOS may not action attempts by the TLXOS installer to change the default UEFI boot order using efibootmgr; on the original Falls City Compute Stick (STCK1A32WFC) this may even crash the BIOS(!).  Consequently, after the install completes you may need to enter BIOS settings using F2 and manually set the correct boot device.  Dell Optiplex BIOSes may have this problem.
• Leaving the PXE boot ROM enabled may clash with the Syslinux boot loader, particularly on NUC BIOSes with boot-from-iSCSI support.  This is likely to be some kind of low memory contention problem, i.e. the BIOS uses too much memory, and overwrites RAM that Syslinux is using, or vice versa.  Unless you are actually using PXE boot, enter BIOS settings and set the PXE boot ROM to Disabled.
• Turning off UEFI boot may prevent Legacy booting of TLXOS because the BIOS doesn't understand GPT partitioning unless UEFI booting is enabled, and (erroneously) pre-emptively decides that the GPT-partitioned TLXOS boot device is unbootable and won't even try to boot it.  This is a common failing of NUC BIOSes.
• On NUCs and Compute Sticks, booting a non-default device using the F10 option may not work, for unknown reasons.  You therefore need to enter BIOS setup using F2 and configure it such that either the USB stick appears first in the UEFI boot order, or select the "boot USB device first" option.
• For reasons we don't understand, old HP/Compaq laptops may be unable to boot TLXOS from an internal disk, even though they can boot the exact same image from a USB disk.  Other than booting RePC Live on a USB stick rather than using an installer, we don't currently have a workaround for this.