I am stuck on the hands-on challenge of week1-day4:  create object relationships. Here is the message I got:

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The field 'Contact__c' either does not exist on the Offer__c object or it is not a lookup relationship.

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I have checked/started-over a few times, and I see the field is there. I don't know what this message means and what I should do.  Can anyone give a hand?  Thanks!