Why can't I update from MacOS High Sierra version 10.13.6? MacBook Pro from Late 2011

High Sierra is the end of the line for that Mac.

You could try installing FireFox 115 ESR. It is no longer getting new features – and in a few months, Mozilla won't even be providing "critical security updates" for it any more. But it might let you browse sites that are giving you trouble when you use High Sierra's version of Safari.

Mozilla Support – Firefox users on macOS 10.12, 10.13 and 10.14 moving to Extended Support Release

Support for 2011 MacBook Pro ENDS with macOS 10.13, High Sierra.

No later MacOS versions are supported by Apple on that model.

A groups of hackers worked on extending some of these older MacBooks, and found that several important features (especially WiFi) rapidly fell away. In looking back at their work, there said that the graphics chips in these older MacBook Pros were so pitiful, they were essentially unusable when pushed to later versions of MacOS that required Metal graphics (10.14 Mojave and later).

Newer Macs, even the Apple-silicon MacBook Air, can run rings around the previous intel "high end" MacBook Pro, provided memory is slightly expanded from the default. if you are mostly consuming content, the latest Apple-silicon iPads are a contender as well.

Currently the oldest MacOS still supported is Monteray which can only take MacBook Pro's from 2015 or later. Though that OS will be starting to lose support once the new OS comes out later this year. You are currently on the highest OS that your MacBook can support. Apple has a list of what every OS can support just search up the Mac OS you're looking for followed y supported devces and it should be the first or second thing to pop up.

macOS 10.14 "Mojave" and higher requires a graphics technology know as "Metal." Hardware support for Metal did not happen until 2012 Macs.

From "System Information" on my 2012 MacBook Pro: