when creating a DKIM record you click the enable button

with the info I can create the dkim DNS record

but in the SM interface that records isn't directly enabled, because DNS check failed.

I think that at the moment I click enable it also test if the record exists. And it doesn't at that moment.
maybe the negative DNS result for the check of the existance of the dkim record is also cached.
For a while DNS will return a does not exist result.
It can take some 5 minutes I noticed before the dkim option can be enabled.

If that's the case then I suggest adding a creation button seperate from the enable.

creation to generate the key to be published in the DNS zone

after creation of the dkim key the Enable key can be clicked which includes the test if the record exists.