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I have a multiple choice question where some combinations of answers should be allowed and one specific combination should be blocked.

Question type: Multiple choice
Which of the following best describes your current situation? (Q1)
Answer options:
A1 – I am employed
A2 – I am studying
A3 – I am currently not employed

Allowed combinations:
A1 + A2
A2 + A3
Any single option
Not allowed combination:

A1 + A3 (employed AND not employed at the same time)

I want to:
the respondent should NOT be able to continue if A1 and A3 are selected together,
a clear validation message should be displayed explaining why this combination is not allowed.

I tried to use Question validation equation with the following condition:
Q1_A1 == "Y" and Q1_A3 == "Y"
but this does not work.

Hi,

Q1_A1 == "Y" and Q1_A3 == "Y"

What does it mean? That ONLY the combination of A1 AND A3 is allowed.
You have to negate this because this combination is NOT allowed
Either - as we learned at school:  The negation of  A==1 AND A=  is  A!=1 OR A!
Or more simple. Negate the whole term !(A==1 AND A=)