It says DKIM is missing even though I have it, why?

It says DKIM is missing even though I have it, why?

The lack of a result can be due to several reasons. DKIM may be misconfigured or missing. You may also be using a unique selector, which prevents us from verifying DKIM. If you want us to be able to verify DKIM, you are welcome to submit a support case with information about which selector you are using.