Wordpress sometimes needs you to update the php version. Outside of a managed hosting, if you’re working with raw Linux, you’re pretty much stuck. Ignoring the official, unhelpful page at https://wordpress.org/support/update-php, what actually has to be done is this:

Using the example of updating from PHP 7.2 to 7.4, and assuming you have installed 7.4 from command-line already, add the line:

AddHandler application/x-httpd-php74 .php

to the .htaccess file in the web root. Now call:

sudo a2dismod php7.2

sudo a2enmod php7.4

sudo systemctl restart apache2

And done.