Excel powerpivot - create relationship between 2 excel tables files located in my des

I have 2 excel tables files.One table has info about sales by country, by model, by date...and the other table has units by date.

Obviously, the common key is "date"....and by creating a relationship, I can add "units" to my combined resulting table.

However, I can't create the relationship. Excel keeps telling me: "the relationship cannot be created because each column contains duplicate values. Select at least one column that contains only unique values".

Can anybody explain how to resolve this in a step-by-step way, since I am not good with excel.
Thanks.

In order to create a relationship between table, a unique key (data set) is required in at least one of the tables. If both your tables have duplicates dates, you will either have to create a unique key, or try a many-to-many relationship between the tables.

The unique Key can be as simple as 1, 2, 3.

To create a many-to-many relationship between the tables, you'll have to go into PowerPivot, click the Manage icon (Excel 365), select Diagram view, the drag and drop a relationship line between shared columns. Google "many to many relationships" for a more detailed explanation with examples.