Referenced Objects setup in Salesforce

I'm having a crazy time here.

I've got an agreement template set up with Opportunity as the Master Object Type.

I want to reference other objects.

I have Account = {!Account.Id}. I have Opportunity = {!Id}. It is fine with that. If I try to put in {!Contract.Id}, then it starts spitting errors about there being no such relation. Even though it is clearly there, a standard field and all visibility being on.

Where am I going wrong here?