SOLVED (TEMPORARILY)
I didn’t ask for Jetpack, and I don’t use it, so this is super frustrating. I literally just set up my WP today, and it was working fine but now I get the same error everyone else does.
I discovered that line 41 calls a file called ‘class.jetpack.php’. Looks like that file is missing in the directory it is supposed to be in, so I downloaded it from here:
https://raw.githubusercontent.com/Automattic/jetpack/master/class.jetpack.php
And then I added it into the wp-content/plugins/jetpack directory. So far, so good.