what is the last OS? I am still running on High Sierra

Depends on the specs of your Mac. Go to Apple menu > About this Mac and tell us the model and year of your Mac. Please do not post the serial #.

You can check here if you meet the system requirements:

How to upgrade to macOS Mojave - Apple Support

How to upgrade to macOS Catalina - Apple Support

MEWHollywood wrote:

MacBook Pro Retina, 15 inch, mid 2014

That is supported by Catalina. Use go64 first and find all 32-bit apps and locate upgrades or replacements or alternatives (as Catalina drops all 32-bit app support), check that your printers and scanners have drivers, and then have a complete backup before upgrading.