When we validate the DKIM record, we employ a few "generic" DKIM keys. Some ESPs, on the other hand, provide you with a very specific key to verify with instead of those generic keys.
Because of this, a text input requesting that you enter the DKIM selection will appear when you click into the mailbox containing the DKIM warning.
DKIM selector is “autogenerated”, so you can input that, or you can just ignore the error, because DKIM is there either way