Circle with line thru it on iMac startup screen

Follow the steps at > If your Mac starts up to a circle with a line through it - Apple Support

Please remember that your iMac is kind of old at this point and there is a chance that Disk Utility cannot repair the HD. This means it is time to consider replacing the iMac, if you arrive at the same conclusion please use one of Apple's free recycling programs, you can find yours clicking and following Apple Recycling Programs. For an iMac that is running Mac OS 11.7 (as your profile shows) that means several things:

• Mac is obsolete, meaning it will not run current versions of Mac OS.
• Mac is considered vintage, meaning parts for the most part are simply not available.
• Mac has no commercial value