Can’t Update PHP to version 8.0 or above

My web site is http://appletechtalk.com. I am using the free version of Simple Catch 3.8.1, with a child theme and it is using PHP 7.4. I have been advised by my hosting company that I should update my PHP version to 8.1. When I updated the PHP version on the hosting site, the formatting of my site gets messed up. The site no longer goes to the edges of the screen, the formatting is changed and only a couple of the posts on my Home page are visible rather than the entire page. Can you help me figure out why I cannot update my PHP version beyond 7.4? Thanks.