Need to scrub my HD (maybe)

I'm new at this, so please stick with amateur terms as much as possible! My hard disk contains a botched Windows 10 installation and Ubuntu 20.04. This was a dual boot system until I tried to do a clean Win10 install and that wrecked everything. Dual boot no longer works. I'm currently running from a USB stick. I'm at the point where I just want to use the entire HD for Ubuntu.

When I try to install Ubuntu, it gets as far as "Detecting file systems". It does its thing for a while and then stops. The only way to terminate the installation is a hard reboot. I'm guessing that it doesn't know how to handle the mess on the HD. What command would I use to wipe and reformat the HD so the Ubuntu installer see what appears to be a new HD? It seems like I need a blunt instrument to do this.