installed but cannot activate

Hi, I am a complete novice but follow instructions reasonably well. I have installed the plug-in to create my staging site but it will not activate. I think it may have something to do with my securi plugin as this is the page that appears when I click activate?
PHP ERROR:array(4) {
[“type”]=>
int(1)
[“message”]=>
string(706) “Uncaught Error: Call to undefined function mysql_real_escape_string() in /var/www/vhosts/dasco.org.uk/httpdocs/wp-content/plugins/sucuri-wp-plugin/sucuri.php:2451
Stack trace:
#0 /var/www/vhosts/dasco.org.uk/httpdocs/wp-includes/class-wp-hook.php(286): sucuri_events_without_actions(”)
#1 /var/www/vhosts/dasco.org.uk/httpdocs/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(NULL, Array)
#2 /var/www/vhosts/dasco.org.uk/httpdocs/wp-includes/plugin.php(465): WP_Hook->do_action(Array)
#3 /var/www/vhosts/dasco.org.uk/httpdocs/wp-admin/admin.php(169): do_action(‘admin_init’)
#4 /var/www/vhosts/dasco.org.uk/httpdocs/wp-admin/plugins.php(10): require_once(‘/var/www/vhosts…’)
#5 {main}
thrown”
[“file”]=>
string(84) “/var/www/vhosts/dasco.org.uk/httpdocs/wp-content/plugins/sucuri-wp-plugin/sucuri.php”
[“line”]=>
int(2451)
}
my php has recently been updated to version 7.1.26, wordpress version 5.1.1 securi version 4.5

I hope this is the correct place to ask for help, I appreciate any guidance you are able to give me.