Help with a custom data rule formula

I'm trying to add a custom data rule for a table that will come up with a message for the user. Can you help me figuring out whats wrong with this formula?

If (
[Not available] = true,
"Please check that your items are available during your event dates. Thank you!"
)

The [Not available] field is a lookup field, so I'm trying to say that if it contains any dates, to show this message. However, I'm getting an error that says that on the left hand side of the operator "=" is of type textlist while the expression true on the right hand side is of type bool. The operator "=" cannot be used with these types of expressions.

Thanks!

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Megan Hardison
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