DUNS Mismatch

"Mismatch" here almost never means the number is wrong. It means Apple looked up your D-U-N-S record and the details it found do not line up, character for character, with what you typed into the enrollment form. Registering with D&B is only half of it. Apple's check reads three fields off that record - Apple's own wording is that the D-U-N-S Number lets it "verify your organization's identity, legal entity status, and address" - and all three have to agree with the enrollment form. What to check, in this order:

• Look up the record yourself first. Apple runs a D-U-N-S lookup at developer.apple.com/enroll/duns-lookup/ and it shows you what Apple sees, not what you think D&B stored. Compare it word by word with your enrollment form.
• Legal name. The usual culprits are the legal form and its position (Ltd, LLC, GmbH, Pvt Ltd, OU), an ampersand versus "and", punctuation, and accented characters. A trading name instead of the registered name will also fail.
• Address. If you moved and only updated the tax office, D&B still has the old one.
• Entity status. Apple will not accept a DBA, a fictitious or trade name, or a branch - its wording is that it does "not accept DBAs, fictitious business names, trade names, or branches." If D&B has you filed as something other than the legal entity itself, that reads as a mismatch too.
• Duplicates. If you applied to D&B more than once you may now have two records, and Apple may be reading the wrong one. That takes longer to untangle than anything else on this list, so avoid creating another one while you wait.

When you do find the wrong field, fix it at D&B, not at Apple - Apple only reads what D&B sends. And then wait before retrying: Apple publishes up to 2 business days for updated D&B information to reach it. Resubmitting the same enrollment the same day simply re-reads the old data and fails again.