Thinkpad T14s BIOS settings - Can't load slackware

I have a Thinkpad t14s on which I recently installed slackware 15. The installation went smoothly. The problem is booting - when the computer turns on, it should recognize the OS and boot to it. Instead, I'm stuck in the select boot device screen. Choosing Slackware only makes the screen flash and then reappear.

I already turned off secureboot. Any ideas on which settings I should try changing?