Charles & Nick,
I've just been thru the process of creating a new domain and stepped thru the ClearOS Documentation HowTo for installing and creating a DKIM key. I also did EXACTLY as per the HowTo and reading your previous post Charles, I figured out where I also was going wrong.

In the HowTo it states

Now you need to update your DNS records. Open the file '/etc/opendkim/keys/mydomain.com/YYYYMM.txt'. In your DNS records, create a new TXT record with a subdomain as the first field in the file which you can just copy. In this case it is “YYYYMM._domainkey”. For TXT Data copy and paste everything between the first and last set of quotes, excluding the first and last quotes and removing the middle quotes and whitespace between them.

I didn't realize the "DNS records, create a new TXT record with a subdomain" were referencing the secure.clearcenter.com/portal/dns_txt.jsp" target="_blank">https://secure.clearcenter.com/portal/dns_txt.jsp where I'd set up my domain. I was thinking this was referencing somewhere on my COS server and was left scratching my head on this for almost a week!

Anyway, screenshots of the HowTo plus the ClearCenter DNS Config TXT tool

I repeated the Howto steps but added a new subdomain "202012._domainkey" using the DNS Config TXT tool on the secure.clearcenter.com/portal. and now I have a working openDKIM key.

I tested the SPF and DKIM key using https://www.mail-tester.com/spf-dkim-check
Everything now checks out and I'm confident I've got a valid SPF and DKIM record for my new Domain and emails should happily reach their destination... although this is still yet to be tested.