The field 'Property__c' either does not exist on the Offer__c object or it is not a Master-Detail relationship.

Hi Vera, 

 Please check that the Property__c field was actually created on the correct object.If the object placement is correct, the next thing to check is Field-Level Security. Even when the field and relationship type are set up properly, if the field isn't visible/accessible to your profile. then you will not found the field.