I have a custom object called Service Engagements, and a related object called Service engagement parts.  I would like to create a validation rule so that when the user is in the service engagements, they are not able to create a Parts Request if the status of the SE is set to 'Ready for Billing'  'Billing Complete'  'Posted'  Complete - No Charge'.

The button for the Parts Request brings them to a visual force page, there is code in the class that is supposed to set this to readonly - true, and then gives a error message.  The problem is that for some strange reason (which I am still looking into) this does not work all the time, it is hit or miss.  I am working with the developer to figure out the issue, it is just taking a while. 

So in the meantime I was going to make a validation rule on the SE Parts request that says if the Service Engagement status is set to 'Ready for Billing'  'Billing Complete'  'Posted'  Complete - No Charge', it won't allow them to save it.  I realize this will be a pain for the user since they have already filled out the info, but they are supposed to check anyhow.

So the Parts Request Object I started a validation rules that looks at the status of the SE.  Service_Engagement__r.Status__c , but I have no idea how to write this (I am very new to this as you can tell).  Any help would be greatly appreciated.  I am guessing I can set this so that they can't save it just with the error message.

Thank you very much for any input.