upgrade OS MacBook Pro 13" 2011

As previous person mentioned and directions to download the High Sierra.

Make sure to use Only Safari to commence the download as Other may not work

Make a Time Machine Backup Before attempting.

Are there any AntiVirus, Disk Cleaner, Optimizers, Defrag, VPNs 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.

I believe the furthest you can go on this mac is macOS High Sierra (10.13.6).

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

Welcome :-)

The real work was done by @Luis S