Unable to use string columns in any way in DirectQuery mode.

Hi guys,

I'm using direct query to get the data from an OLAP cube. The columns Dim_date, Fiscal_year, Fiscal_week, FY, FP, etc are not available in the DAX expression box when typing. This the error i get when i give the expression :

dim_month_1 = MONTH('time'[Dim_date])

The error:  (here the [Dim_date] column is a date column in string format)After spending some time researching i was able to learn that i need to aggregate, so i used the following expression:

=SUM('Time'[FP])

As you can see the column 'FP' is just a value in string format, but it still shoes this error:

And when i try and convert it into a value by using the expression:

=VALUE(SUM('Time'[FP]))

This is the error: 

Please help, i'm new so if i made a silly mistake, forgive me.

Thanks