#plugin version
#PHP 7.1+ doesn’t like & in front of $query parameters in functions remove it and you should be fine.
Example in the source

#Custom theme (old themes that does not support the new PHP 7.1+ version)
#PHP 7.1 doesn’t like & in front of $query parameters in functions.
#In the Themes ->  -> functions.php -> search the code and remove the &
__search_by_title_only( $search, $wp_query ) {

#source: https://generatepress.com/forums/topic/search-function-error/