Call to undefined function create_function()

create_function was removed in PHP 8.0, so your theme does not appear to be compatible with PHP 8.0+. You have several options to resolve this:

1. If your site is down and it is important to get your site up immediately, ask your host to downgrade PHP to 7.4. Just be aware that security support for PHP 7.4 ends in a couple of days, so the sooner you can resolve this the better.
2. Contact the theme author and find out if there is an update available that will resolve this issue, or if they can provide any support for making the theme compatible with PHP 8.0+.
3. If, and only if, your theme is not receiving updates and the theme author is unavailable or can’t help, you can update the affected code like this:

create_function( '$number', 'return 12;' )

Needs to become:

function( $number ) { return 12; }

The PHP documentation has some more examples of converting create_function() into anonymous functions, which is what you need to do.