cannot modify header

Just installed wp-staging and right out of the gate I received a

Warning: Cannot modify header information – headers already sent by (output started at /staging/wp-content/plugins/wp-staging/wp-staging.php:1) in /staging/wp-includes/pluggable.php on line 1210

I believe this is because there is an extra line in you php that is tripping things up. Can you help me understand how to fix