Just wanted to add that the manual installation guide does work! So the “marked solution” is a bit misleading since it literally says: “the manual guide does not work, so use the plugin!”
This is problematic for two reasons:
(1) It’s just wrong, since the manual installation does work. After all, if that statement were true, then clearly the grav developers should take their manual installation guide offline… (And they didn’t since it is correct.)
(2) Some people (like me!) can’t use the plugin, since it literally requires terminal access to the server. I, for example, use grav and have full access to all files, but no access to a terminal, so I can’t use the plugin. (Although I can of course install it using the GUI admin tool, but actually using it still requires a terminal, so it can’t be used via GUI/browser, it only can be installed by it.
So let me give you the actual solution: Carefully double-check that you did painfully follow every single step in that manual installation guide. To cut things short, please read the very last entry (by pamtbaau from Aug’18) of the similar problem discussion, because it mentions some frequently made errors. (I believe that you can ignore all the fancy stuff above since for me the manual installation worked once I figured out my mistake.)
And to motivate my response: I actually had exactly the same error message after following the guide. I thought I followed it perfectly, but after reading that post I just linked I realized a “minor” mistake: When I copied the PHP code I didn’t realize that it mentions the class name (“class Mytheme extends Quark”), so my code literally contained the class name Mytheme instead of “Thenameofmythemethatiactuallydefined”. Once I changed the name the error message disappeared.