Multiple Select not working

Hi, I’m a struggling newbie to coding. I’m trying to get jQuery Select2 working within Wordpress plugin “Smart Grid Layout Design for Contact Form 7”.

I have everything working except it will only allow me to make a single choice. I have checked “Enable Multiple Selection”, clicked on the “PHP” icon, copied the text to the “Functions PHP” file and selected the correct category ID.

When I click on it the correct list appears but it will only store one selection.
It’s probably me being stupid but could someone help me please as I have a deadline and I’m not getting anywhere.
Thank you