A dropdown list is intended to be a list of valid choices that the user can choose from. What you are trying to do, I think, is include "No" in the dropdown list, but if the user then selects "No" then you want to display an error message saying that the user shouldn't have selected "No". That being the case, then why would "No" be in the list in the first place?
If you are trying to convey that Task Complete should either be "yes" or blank, then would using a checkbox be clearer than using a dropdown list? The attached workbook shows that approach.
If you really need your original approach then I'm afraid I know of no way (without VBA) to cause the selection of a dropdown list item to generate a popup error. The closest I can think of would be to have some other cell with a formula that checked B5 for "No" and displayed a message. Something like:
Let me know if either of these options would work for you. If not then maybe someone else will pitch in with an idea.
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