Please understand that WordPress does not support this. WordPress is intended to host a single website on a single domain name.
Your website appears to be configured to be hosted on https://www.carolinedai.infy.uk/. So that URL works. But it’s not configured to work on http://www.carolinedai.co.uk/, so that URL does not work correctly.
If you want to keep using WordPress, I think you’ll need to do the following:
Choose whether your website is hosted on the free subdomain or the custom domain (you can choose only one!).
Configure the website URL in WordPress to use this domain name.
Setup a redirect from the domain you didn’t choose to your new domain name, to ensure no traffic is lost.
We can probably give you more specific instructions if we know more about exactly what you need to do. Also, can you please tell us the Directory on your account that both domains are linked to?
This will make it so that the subdomain will not be linked to the main htdocs directory, but to a separate directory carolinedai.infy.uk/htdocs. This enables you to configure a redirect that only applies to the subdomain, but not your new domain.
Use the Redirects tool in the client area to setup a redirect for your subdomain to redirect it to your custom domain.
When you have completed these steps, WordPress should be fully functional on your new domain and your subdomain will redirect all visitors to it.