My calculation does not work using variable (row context issue or something else?)

How about this for the variable:

Var vNotEligibleStep1 = 
    COUNTROWS(
        FILTER(
            Data,
            Data[Consent] = "Not Eligible"  &&
            Data[Step] = 1
        )
    )

This will return 1 for each row which meets the filtered criteria. You can now keep the switch function clause.

Alternatively you can try:

new measure = SUMX(Data, [Step 1_0819])