Validation Rule to restrict override of Position Name

Team,

We are working on writing the Default and Validation rules for defaulting and validating the Position Name as a combination of Job-Department-Location-Union respectively; while we are able to default it but we aren’t able to fire the validation rule.

Here is what we are trying to achieve:

1. Default the Position Name based on the values selected for Job-Department-Location-Union – Working
2. Question: Can we make Position Name Field as Non-Editable?
3. If not, we decided to implement the validation rule to ensure users aren’t able to overwrite the default values

 Here is what we are using for condition as a POC: