Need help for upgrading the Mac OS on my daughter's iMac 21

My daughter's iMac 21 (Late 2013) is currently running Yosemite 10.10.5 (616 GB free space on HD) The OS needs to be upgraded in general and because her new iPhone asks for a later OS for setup.

When I try downloading Catalina for her computer, I get the message (below) that the link needs to be opening in an application. When I select 'Open Link' to iTunes (12.8.1.3) on the iMac, nothing happens.

I'm stuck at this point. Any advice and direction will be much appreciated by me . . . and my daughter!

Stay safe.

Ed in Brooklyn