In the tabular database model, you can only create a relationship between two tables when at least one of the columns being used to create the relationship has unique values. It seems like both of your date fields have duplicates values.
You need to deteremine which of these two date columns in your two tables should have a unique values. Then use the Power BI Desktop query tool to de-duplify these duplicate date values so that the query outputs a table with unique column values. After that, you should be able to create the relationship.