You see that error because something is sending output (even space characters) before the header-location command. The error message tells you where the output is coming from. To fix, you’d have to edit that file – not a wise choice to edit core files, because the next update will overwrite your changes.
Assuming your plugin has a need to redirect, then you should use the wp_redirect() function (see here https://developer.wordpress.org/reference/functions/wp_redirect/ ).
Remember to put a exit; after the wp_redirect() to ensure any code following the wp_redirect is not executed – as the function’s docs explain.