The end of the error message
No such file or directory in [...] should tell you where it is currently looking for that file. From there you should be able to work out if you need to tell PHP to go up or down a directory from where it is looking.
When dealing with local files like using
include I'll often use the
$str = file_get_contents(__DIR__ . 'employees.json');
You could also echo out
__DIR__ and work from there where your file is relative to that path.