I can’t get my OS X to install to my Mack
I can’t install OS X on my desk top. It says it’s unavailable in the App Store
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Which model and which model year Mac? With both the model and the model year, we can determine which macOS versions are available for this Mac. The Apple menu > About This Mac details will show both the model and model year.
Which version of macOS are you trying to obtain from the app store?
Here is how to get older versions: How to get old versions of macOS - Apple Support
El Capitan installation requires some extra steps. These steps are described in the article.