To avoid a DKIM failure, the cPanel does not sign the outbound email of the domain with the DKIM signature if the DKIM key is invalid. That is possibly the cause why cPanel DKIM does not work.
As the first step, you must ensure a correct DKIM record exists within the DNS of the domain in question. You can use the Email Deliverability in cPanel for this. In addition, you can use the Email Deliverability in WHM if it is for searching DKIM for the server's hostname.
If an invalid DKIM key is present in either case, the abovementioned issue occurs. So, make sure your DKIM record is valid using a third-party tool like this.
Read more here.
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