Wordpress Fatal error: Class 'WP_List_Table' not found in[...] Explanation
This error is quite easy to be fixed, the ideea is to understand the reason why this error shows up.
You probably want to create a new page in admin within a new plugin to list some custom things, because this is the most common place where this fatal error occurs.
The main ideea is to understand that at the point where you are trying to extend the WP_List_Table class this class has not been included by the Wordpress flow.
Try to include the file that contain your class later in the flow.
You also can use this PHP native function to check what classes are already included until this PHP function is called ( function is: get_declared_classes() )
That should do it.