Upgrade to the newest version of OS X
https://www.apple.com/macos/how-to-upgrade/

Click the following link. It will list a many of the newest versions of OS X. Click on the newest version, it will take you to the Apple Store. Click Get. If it works then that version is compatible with your system. If it is not compatible, then try an older version. https://support.apple.com/en-us/HT211683

https://www.uvm.edu/it/kb/article/outdated-macos-and-how-to-upgrade/