“MacOSBigSurInstall11.6.5.pkg.download doesn’t appear to be a valid download file."

[TechNewbie2]

This is the message Safari provides in their window box after double clicking on the download from this website. I don't want to upgrade to Monterrey just yet due to wanting to isolate a potential problem in a separate volume already created for Big Sur's installation. Apple Store, of course, only supplies Monterrey at this time and the links on their pages of previous O/S's are broken. I just spoke with a sr tech there who had no viable recourse. I don't know if I can create a bootable USB. Thanks for the input.

 

[TechNewbie2]

It's on my 2017 iMac and I am currently running Catalina.