I need help to get back into my MacBook Pro.

I need help to get back into my MacBook Pro. I attempted to upgrade to High Sierra and my guess is that it is just on the borderline of acceptable hardware for the program.

The program attempts to install but then the screen says: "macOS could not be installed on your computer" And then: "An error occurred installing macOS/ To use Apple Diagnostics to check your Mac hardware, shut down, press the power button, and immediately hold the "D" key until diagnostics begins." "Quit the installer to restart your computer and try again."

If I exit the macOS installer it asks me to choose a startup disk - what do I enter?

Turning the MacBook Pro off and then on again just repeats the process with seemingly no way to open the computer.