As you can see in the error, you are trying to compare "true" to an enumeration and you can't. You have to change the attribute to a boolean or you have to compare the value with a value from the enum like this:
This means that your expression is not correct. You should write it like this: "true" - lowercase.
But this only applies if the attribute is of type Boolean.
Hm, this didn’t do it for me…
I’m in the expression engine, I have called from the validation part of a text box
Got it, thanks!