How to calculate rolling 5 week average ?

Hi newlearnpbi123, here is an easy method:

Rolling average = 
VAR _NumberOfPeriods = 2                                    //change it to 5 or any other number
VAR _MinPeriod = MIN( 'Table'[Week] )                       //get the minimum week for current filter context
VAR _NumPreviousWeeks =                                     //this measure is required to start calcualtion only when you get last N weeks
    COUNTROWS(
        CALCULATETABLE(
            VALUES( 'Table'[Week] ),
            'Table'[Week] < _MinPeriod
        )
    )
RETURN
    IF(
        _NumPreviousWeeks < _NumberOfPeriods,
        BLANK(),
        CALCULATE(
            AVERAGE( 'Table'[Value] ),                          //calculate the average
            'Table'[Week] >= _MinPeriod - _NumberOfPeriods,     //define min week
            'Table'[Week] < _MinPeriod                          //define max week
        )
    )

Here is the output:

Feel free to adjust formula parameters as needed.

Good luck!

You can upload sample pbix used by for the example here. If same calculation provides blank(), than there is something in your semantic model that should be taken into account. However, I hope that by having this pbix as example and making research online you can find the answer! If not, please share your pbix sample with your semantic model.