Calculating Consecutive Years Active

Please try this measure expression in a table visual with your Fiscal Year column, replace Donors with your actual table name.

Consec Yrs =
VAR vMaxYear =
    MAX ( Donors[Fiscal Year] )
VAR vThisCustomer =
    MIN ( Donors[Customer ID] )
VAR vLatestNotDonation =
    MAXX (
        FILTER (
            ALL ( Donors[Fiscal Year] ),
            ISBLANK (
                CALCULATE (
                    COUNTROWS ( Donors ),
                    Donors[Customer ID] = vThisCustomer
                )
            )
        ),
        Donors[Fiscal Year]
    )
VAR vLatestIfBlank =
    IF (
        ISBLANK ( vLatestNotDonation ),
        MINX (
            ALL ( Donors[Fiscal Year] ),
            Donors[Fiscal Year]
        ) - 1,
        vLatestNotDonation
    )
RETURN
    IF (
        vMaxYear = 2020,
        vMaxYear - vLatestIfBlank
    )

Pat