calculate days count between 2 dates

Wadda7AboUdai 
Yes that is because in expected result that you have presented, the transaction number is part of the filter context. If you place the transaction number in the result summary table it should work. However, the following should work in both senarios.

Days Count = 
SUMX ( 
    SUMMARIZE ( 
        'inventtransn',
        'inventtransn'[item],
        'inventtransn'[transaction number]
    ),
    VAR PurchaseDate =
        CALCULATE (
            MIN ( 'inventtransn'[DATEPHYSICAL] ),
            'inventtransn'[REFERENCECATEGORY] = 3
        )
    VAR TransferDate =
        CALCULATE (
            MAX ( 'inventtransn'[DATEPHYSICAL] ),
            'inventtransn'[REFERENCECATEGORY] = 22
        )
    RETURN
        IF (
            NOT ISBLANK ( TransferDate ),
            DATEDIFF ( PurchaseDate, TransferDate, DAY )
        )
)