Thank you for bringing this to the Community. This question has been answered in another post found here. Let me know if you have any questions about the information provided there.
If the following troubleshooting steps don't allow you to add your IBAN, then I would recommend contacting Shopify Support.
Can you confirm that your IBAN is for a banking institution physically located in the Republic of Ireland (where you are located). "Virtual" banks, companies that mimic bank accounts in certain locations (like Payoneer accounts), or banks located in a different country can't be supported.
If the error doesn't hint at what needs to be fixed, and the IBAN is from a banking institution physically located in the Republic of Ireland, then we'll want you to try to update this using Google Chromes Incognito mode.
You can do this by logging into your store through the help center, searching your question/issue, then selecting "continue" at the bottom of the page that displays the search results.
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