If you see DKIM is failing for your emails, firstly we need to check for the public key. We can check on multiple websites like whatsmydns.com or mxtoolbox. We have to check the TXT record for that particular domain.

The command to check the DKIM entry is <selectorID>._domainkey.<domain> . This will help to check if the DKIM key is present or not.
For Example - if the selector is emm and domain is netcorecloud.com then check TXT record for emm._domainkey.netcorecloud.com

So if the DKIM key is present, DKIM key validation to be done. You check it on - https://dkimcore.org/tools/keycheck.html - This will show if there is any error in public key.

Another way of validating DKIM error is to check on Google Postmaster whether it is passing or not. If DKIM success rate is 99-100% and you still see the error after checking all the validations above then please test it again after an hour and if the issue is still there then contact DE.