When downgrading to to 5.4 or 5.5 it shows – Your server is running PHP version 5.4.45 but WordPress 5.7.2 requires at least 5.6.20.
So I have used 5.6 for you – it should be working.
Also please note that MySQL extension was removed as of PHP 7.0. So you’re seeing this error, it could be due to old code that needs to be updated. You will need to upgrade the core WordPress files to the current version, as well as any plugins your site is using. So after upgrading everything we could try out 7.4