I can't Install Mojave

I have a 2013 iMac 27". with 32gigs of ram.

The Mojave installer gets to "select disk to install" I click Macintosh HD Which has about 300gigs of free space and it says "This disk is used for iMac backups" and it will not allow me to proceed any further.

Another user suggested using startup with CMD-Control-r-p which I did but still no success.

My machine does not back up to the Macintosh HD. I have time capsule setup to run on a 2TB HD I have off of my AirPort Extreme.

Any suggestions?