Welcome to the Forum Otto Wirke!
Data Validation does not support formulas/cell references in the messages. You would have to remove your data validation rule and use VBA. Something like this:
I have not tested this but would be happy to do so if you provide your file. The paper clip icon does not work for attachments. To attach a file, under the text box where you type your reply click the Go Advanced button. On the next screen scroll down and click on Manage Attachments, which will show a pop-up window to Select and Upload a file. Then close the window.
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