Can't create relationship between 2 columns

I have 2 tables in Power BI Desktop.

RowId, Timestamp, ProductName, ProductSales1

RowId, Timestamp, ProductName, SalesAmount1, SalesAmount2

I tries to make relationshop between Timestamps, but failed. Then I tried between ProductNames.

"You can't create relationship between 2 columsn because one of the columns must have unique values"

What does it mean?