What is the latest version of OS I can install on a Mac Mid 2011?

With the good information supplied by @ D.I. Johnson

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install. Best results use Safari to commence the download as Others may not work.

Make a Time Machine Backup  it very useful and can be used to Revert to Previous Working macOS.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation

A late-2009 thru mid-2011 Mac can be upgraded to High Sierra 10.13.6 but nothing newer.

See: https://support.apple.com/en-us/HT211683

You're welcome.