Theme folder doesnt exist

I’m giving possible cases from the top of my head:

Case 1: May be it’s related to realpath() function. realpath() acts differently in different OS. Also, it returns false if the web server user doesn’t have executable permission on the corresponding directory. There are some other problems too, so better try this:

// change it based on the location of wp-config.php file
define( 'WP_CONTENT_DIR', dirname( __FILE__ ) . '/content' );

Case 2: May be the sage-master theme directory actually doesn’t exist or it’s not accessible by the web server user.

Case 3: May be the directories exist but have character case different. Unix systems have case sensitive file system, but windows is case insensitive. So it may work on windows but not on Unix for this reason.

There may be other issues, so better activate debugging in WordPress and check what errors you get.