How to make Salary Amount field Read Only based on Grade Step on Assignment

Summary:

We have a requirement to make Salary Amount field Read Only if value present in Grade Step in assignment, If Grade Step is blank we need to make Salary Amount field to be editable. I am creating a rule for it, trying to figure out exact assignment Grade Step field but not able to find it, can anyone help me how to find it?

The below condition is not working:

Please help with how to write a condition for my requirement.

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