Plugin cant activate

I get the following error when trying to activate the plugin:

Fatal error: Uncaught TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback, non-static method WAC_Authority_Checker::wac_wpa_install() cannot be called statically in /usr/www/users/pragmckbst/wp-includes/class-wp-hook.php:307 Stack trace: #0 /usr/www/users/pragmckbst/wp-includes/class-wp-hook.php(331): WP_Hook->apply_filters(”, Array) #1 /usr/www/users/pragmckbst/wp-includes/plugin.php(476): WP_Hook->do_action(Array) #2 /usr/www/users/pragmckbst/wp-admin/plugins.php(194): do_action(‘activate_websit…’) #3 {main} thrown in /usr/www/users/pragmckbst/wp-includes/class-wp-hook.php on line 307