DAX comparison operations do not support comparing values of type Date with values of type Text

I have found the solution for this.

here is the piece of code in the DAX code which caused issue.

before fixing: 

        CALCULATETABLE(
            VALUES(Sales[Customer ID]),
            FILTER(
                ALL(Sales),
                Sales[Purchase Date] >= _DateBeforeNDays & Sales[Purchase Date] <= _EndOfMonthDate
            )
        )

After fixing: 

        CALCULATETABLE(
            VALUES(Sales[Customer ID]),
            FILTER(
                ALL(Sales),
                Sales[Purchase Date] >= _DateBeforeNDays && Sales[Purchase Date] <= _EndOfMonthDate
            )
        )

 why the error caused in the first place?

--> if you see in the first code, instead of using && operator I was just using & operator. and that was the only reason for the error.