[SalesForce] SOQL Error “Didn’t understand relationship ‘Addresses__r’ in field path…”

I am going to make some assumptions.

First, I assume that Additional_Contract_Item__c is a custom object that is a "detail" of the Contract object.

Second, I assume tat you want to select all of the Additional_Contract_Item__c records that are related to a single Contract record.

So every master-detail relationship (and every relationship) has two names. One is the relationship from the detail to the master, and one how the master relates to the detail. For instance, in a Parent-Child relationship, to get from the Parent to the Child, you need to access the "Children". To get from the Child to the Parent, you need to access the "Parent".

You seem to be a little confused in how these inner joins work (which is totally reasonable - they're confusing). If I had a Parent__c object, and a Child__c object, with the relationship from Parent__c to Child__c called Children__r, and I wanted all of the children of a parent, I'd do something like this:

SELECT Id, Name, 
      (SELECT Id, 
              Name, 
              Child_Information__c
       FROM Children__r)
FROM Parent__c

To apply this in your case, you'd need to do something like:

SELECT ...,
      (SELECT ...
       FROM Additional_Contract_Items__r)
FROM Contract
WHERE id = :id limit 1

But you need to figure out what the relationship from Contract to Additional_Contract_Item__c is (I am assuming it's called Additional_Contract_Items__r, which would be the default).

However!

I'm not entirely clear why you are even querying on the Contract record. Unless I'm missing something, that's more than you'd need. Since you don't seem to be doing anything with the Contract record itself, you can rethink this query as "I want a set of Additional_Contract_Item__c records that are children of a Contract". Then you can get rid of the join altogether:

SELECT Id, Contract__c, Contract__r.Item_Description__c,
       Contract__r.Quantity__c, Contract__r.Total__c, 
       Contract__r.Unit_Price__c
FROM Additional_Contract_Item__c
WHERE Contract__c = :id

The one last assumption I am making here is that Item_Description__c, Quantity__c, and so on, are fields on the Contract record, not the Additional_Contract_Item__c (in which case the data will be the same in each row). If instead, the fields are on the Additional_Contract_Item__c record, you should delete all the mentions of Contract__r. above.

If you made this custom object then the field for account is also custom so the query should be like this:

Select id, Account__r.Type from opportunity__c