Do you have Evri selected as a postal option? Deselect Evri, as they are grouping the Chan. Isles as part of Europe, and causing this issue.
Or, check your site preferences that Europe is not selected as somewhere you do not post to.
If this is an individual buyer experiencing this issue, add them to your buyer exemption list, it may help.
You are not alone experiencing this. 🙄
@ws.4857