Which forms designer are you using?

Try using this as your formula: isNullOrEmpty(DoYouKnowYourSolution)

I think "Responsive."  Not the "Classic."

Hi  @Underdog  

@bamaeric  has provide the answer your question. His answer works better when you have more options for the "DoYouKnowYourSolution" Choice control 

Alternative answer:

DoYouKnowYourSolution == ""

//This should also work if there is more choices

This works for 2 choices. You will need to add more conditions when you add choices. 

Not (DoYouKnowYourSolution == "Yes" ||  DoYouKnowYourSolution == "No" )

I believe this is what you wanted to achieve

Not(or(contains(DoYouKnowYoursolution, "Yes"), contains(DoYouKnowYoursolution, "No"))) 

// 1. Extra OR

// OR Runtime Function => or(boolean1, boolean2)

// This will take 2 conditions. If you have more that 2 conditions, use this 'or'  ||

// (boolean1 || boolean2 || boolean3 || boolean4)

// 2. Unbalanced Parentheses

// 5 left Parentheses "(" vs 2 right Parentheses ")"

This should also work

Not(contains(DoYouKnowYoursolution, "Yes") || contains(DoYouKnowYoursolution, "No")) 

Cheers

I chose to use:

Not(DoYouKnowYourSolution == "Yes" ||  DoYouKnowYourSolution == "No" )

I felt it would be the simplest for folks here to understand when I leave in a couple weeks.  I had to take the space out after "Not", as it was causing an error. 

Thank you for your help!