I guess your date column name is "dt". Your input format for the date "06-08-2000" (dd-mm-yyyy your case) is not a recognizable date format in power query. So you have to create a actual date formatted value first from your input date. Then with some couple of more steps you will get your required output
Step-1: Create a new column "new_date" in power query as below-

Step-2: Change the data type of newly created column "new_date" to "Date" type
Step-3: Go back to report by clicking "Close & Apply" button.
Step-4: New create a custom column using following DAX in the report
custom_date = FORMAT(your_table_name[new_date],"dd-mmm-yyyy")
Now you have your required formatted date.