I just wanted to mention that if anyone is stuck on the requirements screen, you may be getting a simple error message that doesn’t really help, because the requirement is a multi-function requirement, so I wanted to share this one-liner:
Open this file: concrete/src/Foundation/Environment/FunctionInspector.php
Before the last line in the functionAvailable method, before it sends the result, add this line:
print $functionName . ($result ? ':Success' : ':Failed') . "<br />";
Now go back and refresh the browser. You’ll see that some of the requirements (especially Image Manipulation) needs multiple functions to be available. Adding this line to the file will tell you exactly which function is failing.
There may be a log of this somewhere, I’m new to Concrete CMS, hence being stuck on the requirements screen. 
Maybe someone will find it useful. Happy new year! 