Hi Tim, Pavan and Herman,
First of all thank you for your help. I have tried all of your recommendations and also seen the video but it still doesn't work and I get the same error. In order to discard errors I have built my own AD server. I explain the things I have checked:
- ClearPass points to a valid FQDN of the domain controller as said by Pavan. This is the ouput of the command "network nslookup <domain controller>"

- For his recommendation "Make sure that Clearpass FQDN is resolvable from the DC. If not, we need to add an entry to the DNS server." My ClearPass didn't have a configured FQDN, only its IP address and hostname, so I think that wasn't the problem. Anyway and because I didn't know if ClearPass FQDN was mandatory to join AD domain I configured one and added an entry to the DNS server. Then I have confirmed Clearpass FQDN is resolvable from the DC:In CPPM
In DC
- For Herman's recommendation "Split DNS issues, make sure that ClearPass is configured to query the DNS server on the domain controller (when properly setup with full DNS forwarding, a DNS server that forwards to the domain controller will work as long as all domain DNS records are resolvable)". I have only Primary DNS configured in ClearPass which is the DC IP address.
- I am sure both CPPM and DC are time synced. To be sure I have configured the DC as NTP server for CPPM as said by Tim/Herman.
With all of that, the error still is the same:

What can I do guys? Thanks again.
Regards,
Julián