An "A Entry" is a type of DNS record that tells computers a hostname's IP address. Your server's hostname must have an A entry for everything to function properly.
When you complete the Initial Setup of WHM or change the hostname in WHM, you're given the opportunity to add an A entry for your hostname. If you haven't done that, or if your hostname has changed, here's how.
1) Locate the DNS Functions category.

2) Click Add an A Entry for your Hostname.

3) Confirm that the IP address is the correct one to use for the hostname.

4) Then, click Add Entry.
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If a DNS entry exists already, you will see the message below.

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Otherwise, you should receive an OK message.

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This is the end of the tutorial. You now know how to add an A entry for your server's hostname using WHM.