This means your domain is missing a MX record. If you want to receive emails to your domain then it's very important that your domain has a MX record.
How to fix:
If you are not using an external email provider like Gmail, Zoho Mail, AWS Workmail or similar then the first thing you need to do is create an A or a CNAME record called "mail", for example. It can be called whatever you want. Then create a MX record and enter your newly created subdomain as a value. For priority, you can set 0. If you are using an external mail provider then create a new MX record and enter values that your mail provider provided.