Fatal error after installing Easy Popular Posts

After installing and activating Easy Popular Posts plugin, I only got one post listed in the widget. I got a message saying something about code in easy-popular-posts.php on line #???. So I went to the plugin page and read:

=Specific number of results= If you would like to return a specific number of results as your maximum:

popularPosts(‘count=10’);

so I added this code to the php file in a space at the same line indicated, after disactivating the plugin.

Now I can’t event view the website, nor the WordPress dashboard. All I get is this message:

Fatal error: Call to undefined function get_userdata() in /var/alternc/html/c/cumuluspress/www/southsudaninfo/wordpress_new/wp-includes/query.php on line 2852

I am not an expert coder but I have managed to change code in the past to correct problems, but now I have no idea what to do.

visit: https://southsudaninfo.net