you had a single certificate issued on September 13 that covers both
www.mathtutortime.com. The renewal message that you got for Let's Encrypt probably related to a certificate that was just for
mathtutortime.com (because the renewal messages are sent if you ever have any expiring certificate for which a 1-1 name match replacement wasn't issued! there's a note in the renewal e-mail itself explaining this).
Yesterday you issued separate certificates just for
mathtutortime.com (two of each). Currently, your site is only using the new
www.mathtutortime.com certificate, even when accessed as
mathtutortime.com—hence the error.
You should issue a single new certificate for both names (following the September 13 certificate example) and use that, and then ignore future warnings about the expiry of certificates that you no longer use. Alternatively, you can use the new separate certificates you have, if you update the virtual host configuration on your server to properly refer to both of them.