Problem in The Relationships

The reason is that Power BI’s relationships only filter one way (from dimension -> fact).
So when you filter Site, it affects the fact table, but that filter doesn’t flow back to the Operational Unit dimension.
Both Site and Operational Unit are dimensions , so the filter doesn’t move between them by default.

Count of Operational Units = //Try this and put in the card
CALCULATE(
    DISTINCTCOUNT('Dim_OperationalUnit'[OperationalUnit_id]),
    TREATAS(
        VALUES('Dim_Site_Code'[Site_Code]),
        'Fact_Email'[Site_Code]
    )
)

ahmedshalabyy12 mh2587 provided great solution but here are the alternate approaches:

Count of Operational Units = //Try this and put in the card
DISTINCTCOUNT('Fact Table'[OperationalUnit_id])


//2nd approach:
CALCULATE ( 
     COUNTROWS ( 'Dim Operational Unit' ),
     CROSSFILTER ( FactTable[OperationalUnitId], 'Dim Operational'[OperationalUnitId], BOTH )
)

Basically, in the 2nd measure (above), you are creating a fact table to filter the operational dimension table.