Help install on 2009 iMac

Hello 👋 I was recently gifted a 2009 iMac (model hardware 9.1) currently running OS Yosemite 10.10.5. I was about to give up when I found this awesome tool. I am stuck on the install, it keeps saying local disk not found. Please see screen shots. I’ve tried repairing disk permissions on the HD and I created the USD disk but still not working. Any ideas? Thank you! P.S. I know nothing about computers so please suggest steps like explaining to grandma. 👵