Since you say nothing about what exact Mac/model/year/size you have or what macOS you are trying to install or what macOS you currently have on the machine...
You can look over the specs and see where you fit in—
macOS 10.14 Mojave - Technical Specifications:
https://support.apple.com/kb/SP777
Upgrade to macOS Mojave - Official Apple Support
https://support.apple.com/macos/mojave
https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12
MacBook (Early 2015 or newer)
MacBook Air (Mid 2012 or newer)
MacBook Pro (Mid 2012 or newer)
Mac mini (Late 2012 or newer)
iMac (Late 2012 or newer)
iMac Pro (2017)
Mac Pro (Late 2013; Mid 2010 and Mid 2012 models with recommended Metal-capable graphics cards)
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macOS 10.13 High Sierra - Technical Specifications:
https://support.apple.com/kb/SP765?locale=en_US
How to upgrade to macOS High Sierra
https://support.apple.com/en-us/HT208969
MacBook (Late 2009 or newer)
MacBook Pro (Mid 2010 or newer)
MacBook Air (Late 2010 or newer)
Mac mini (Mid 2010 or newer)
iMac (Late 2009 or newer)
Mac Pro (Mid 2010 or newer)
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If you can run macOS 10.12 Sierra, you can run macOS 10.13 High Sierra
https://itunes.apple.com/us/app/macos-sierra/id1127487414?ls=1&mt=12
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OS X 10.11 El Capitan - Technical Specifications:
https://support.apple.com/kb/SP728
How to upgrade to OS X El Capitan - Apple Support
https://support.apple.com/en-us/HT206886
MacBook (Early 2015)
MacBook (Late 2008 Aluminum, or Early 2009 or newer)
MacBook Pro (Mid/Late 2007 or newer)
MacBook Air (Late 2008 or newer)
Mac mini (Early 2009 or newer)
iMac (Mid 2007 or newer)
Mac Pro (Early 2008 or newer)
Xserve (Early 2009)