Hello Yvette-
I’m sorry to hear about the trouble installing a new theme. If you’ve already updated WordPress to version 5.5 and your hosting account’s PHP version (5.6 is the current minimum for WordPress core, though some themes require at least 7.1 or 7.2), then that is definitely a strange issue.
The site you sent over privately doesn’t look like it’s running WordPress at all, it looks like it may be manually coded, and the phpinfo.php file on that domain shows PHP 7.4.
To troubleshoot further, can you navigate to Tools > Site Health and switch to the Info tab, then click Copy site info to clipboard, and then paste the results here so I can take a closer look?