Hi there,
Sorry this update is not working on your device. You can follow the comment Upgrade 5.0.6 to 5.1.0 failed, ASUS frozen at Bios to boot with previous version.
I believe the step 3 is the GRUB menu you see. In the GRUB menu, you can choose “Advanced …” , then the second item should be “Endless OS 5.0.6 …” (for example) and boot with it.
After you boot into the system. Please help us have the debug log as the step 4 & 5 in the comment. Thanks!