I added a domain and then I deleted it by accident. When I tried to add it again, I received Error : .
 
What do I do to fix it?

This error code corresponds to DNS Zone already exists. This could be resolved by removing existing DNS zone from System32/DNS location on your server.
 
For checking HC related errors you can search in ErrorCodes.xml on your HC installation path. I checked against your given code and it was DNS related issue. We were given this solution of DNS zone deletion from System32\DNS folder by HC Support team so I hope this would work in your case too.

i am currently using hcnix not hc8

If you are using HCnix then please check if entry of DNS exists in your Bind DNS server or not.

no it does not.

supertoro

I added a domain and then I deleted it by accident. When I tried to add it again, I received Error : .

What do I do to fix it?

Did you remove domain using HCnix panel or manually from apache ? Because if someone delete domain using control panel then they should be able to recreate it.
Are you able to add any other domain in HCnix panel ? Or try to add problem website under new user.
You can also open a support ticket  tech member will debug the issue in your panel.

I got it fixed. 

Yes, I removed it from the HCnix panel. When I tried adding it again. I received the error. It was only for that one particular domain. Even under a new user, I had the same problem.
 
It was a simple fix.
Turns out the file db.crestdns.com was not deleted and still left on the server under /var/named/chroot/var/named
After deleting it, I as able to re-add my domain.