macOS Utilities Not Allowing Me to Reinstall macOS

Got a rebuilt Mac Pro. I did something in "Advanced Options" when trying to change Admin name and password, which locked me out of Admin. As a remedy I am attempting to reinstall High Sierra using the macOS Utilities. I followed the instructions by erasing my HD, selecting "Reinstall macOS", and clicking on "Continue". I am given instructions, "To install macOS High Sierra, a firmware update is required. To complete the update, click "Shut Down" and wait for your Mac Pro to power off." However, nothing happens when I click on "Shut Down." I have access to the internet.

Is there another way to install a new OS on this mac?