I don't get this joke

I think part of the joke is that the guy is also outing himself as not knowing how the shift key works. Like, he’s hitting the key, trying to type a capital letter or @ sign, but because he’s not holding it while he hits the second keystroke, he’s convinced it doesn’t actually do anything.

It was probably just something in the keyboard layout section of the OS that was set to UK for some reason. I don’t know how old your computer was or what options the OS had, but I know I’ve changed keyboard layouts for European keyboards back in the early 00s to an American layout when I lived abroad somewhere in system preferences.

Ah, I see what you did there! 2Si_Amigo