Mark as date

[Anonymous]

I would like to mark as date a column to create a map too show the variation of the defect number. 

I can't do it : when I select the column I want as date an error appears: 

"The date column must have unique values. The date column can't have differences between dates". I don't get this message. 

What can I do? 

Thanks for your help

[V-lianl-msft]

You could create a calculated table with CALENDAR function or CALENDARAUTO function.
Calendar table =  CALENDAR(MIN('table'[date]), MAX('table'[date]))

Then you could create the relationships between the Calendar table and your original table.

[PaulDBrown · Accepted solution]

You need to create a date table with unique continuous vales of dates covering the range of dates in the model. You can do this in Power Query or by creating a new table using the DAX functions CALENDAR or CALENDARAUTO. 
Do you have sample data or PBIX file?