Cannot successfully enter a condition for a dynamic column

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Hello experts,

I should configure a dynamic column condition, but I incur in the attached error message.

The highlighted field is a custom list column and after the equal symbol I should enter the code assigned to the lookup value.

However, I get the attached error message? Moreover, the highlighted field is enabled in the plan.

What could it depend on?

Thanks in advance for your time!

Kind Regards,

Valerio