I just went through this, not sure how it happened but I finally fixed the pluggable.php fatal error!
Very simply put, when you get this error 99% of the time it means that there are extra spaces in places where there should not be.
First thing to do is check what type of error you are getting and see which file it’s pointing too.
It’s most likely pointing to wp-settings.php. Open this file and scroll all the way to the bottom. DELETE any extra lines after the closing php tag “?>”.
That should fix it. If it doesn’t start looking through your important files like wp-config.php, etc. and repeat the steps.
Hope that helps!