Not enough information to locate the problem.
Does the BIOS detect your PenDriva? I mean, do you see him, his markings in any BIOS tab?
If you use UEFI mode, if the Bootable Disk has been badly made, or there is an incompatible system, UEFI will not show USB Disk or PenDriva in the boot options, but it usually appears somewhere in the BIOS.
If the USB system has a 32-bit UEFI version and the supported version on your computer is 64-bit (and vice versa), the system installation will not start and the UEFI file will not be detected or an incompatibility error will appear.
questions:
What system are you trying to install and in what version?
What options do you choose in RUFUS to create a bootable USB?
What is this flash drive? Some flash drives have problems being detected when turning on the computer (although it won't help, because the only way to determine if you have this problem is to use a different PenDriva or USB drive).
Boot Legacy will help if you are installing a system incompatible with UEFI. Disabling Secure Boot will only help if the system you are installing is UEFI compatible, but it is some older version of one system, or a test version of some exotic system that does not have the appropriate certificates to run in Secure Boot. If you are installing a modern system, no matter if it is Windows or Linux, they are mostly compatible with full UEFI and Secure Boot.