The theme requires at least PHP 7.3 or above because this is what WordPress itself recommends.
Updating Your PHP Version
In most cases you cannot update the PHP version yourself and need to contact your host about this. The upgrade process is easy and should be something your host can do for you without impacting your website or charging you a fee. Here's an email you can send to your hosting company:
I'm running a site on one of your servers and WordPress has listed PHP 7.2 as the recommended version on their requirements page: https://wordpress.org/about/requirements/
Can you please let me know if my hosting supports PHP 7.2 or higher and how I can upgrade?
Looking forward to your reply.
If you have a VPS server, see How to upgrade from PHP 5.
My Host Doesn't Support PHP 5.6
If your host doesn't support PHP 5.6 or higher, you will need to find a host that does. We recommend DreamHost which supports PHP 5.6 or higher. If you contact another host, be sure to ask them which PHP version your website will run on before purchasing.