Objective: The object you created for the previous challenge is pretty handy. Imagine how much more useful it would be if brokers could specify which client made an offer and which property the client wants to buy. Add two relationships to the Offer object so brokers can capture this data in Salesforce. Create a Master-Detail relationship with the Property object and a Lookup relationship with the Contact object.
On the Offer object, add a Master-Detail relationship to the Property object named Property

Error Message Received: You cannot create a new Master-Detail relationship on an existing custom object if records already exist. You must first create a Lookup relationship, populate the lookup field with data in all records, and then change the relationship type to Master-Detail. 

please help me ....

Two team members need to view and update Accounts, but only one should be able to see and edit the Rating field. Create one profile and one permission set with field level security to solve for this use case.The profile must be named 'Basic Account User' and result in an API name of 'Basic_Account_User'. It should use the 'Salesforce' user license type.
The permission set must be named ‘Account Rating’ and result in an API name of 'Account_Rating'.Check Challenge