Philosophical point first - The question is not 'How do I tell the user they have done something wrong', the question is 'How do I stop the user doing the wrong thing?'
In this case you need to disable the submit button until the user has selected something from the combo box.
Assuming the Submit button is disabled in the form design, that the button is called btnSubmit and the combo box is called cmbChoice, you'll need to place the following code in the combo box's AfterUpdate event:
If nz(me.cmbChoice,0)<>0 then
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