Issues with IF and Switch statement

You don't necessarily need to use SUMX at the transaction-level granularity but you do need to iterate over multiple exchange rates somehow.

You could try this instead:

REPORTING Amount =
SUMX (
    VALUES ( Transactions[REPORTINGCURRENCY] ),
    SWITCH (
        Transactions[REPORTINGCURRENCY],
        "USD", 0.376,
        "GBP", 0.475,
        1
    )
        * CALCULATE (
            SUM ( Transactions[ACCTINGCURRAMOUNT] )
        )
)

Or you could manually iterate over each currency like this:

REPORTING Amount =
CALCULATE (
    SUM ( Transactions[ACCTINGCURRAMOUNT] ),
    KEEPFILTERS ( NOT Transactions[REPORTINGCURRENCY] IN { "USD", "GBP" } )
)
    + CALCULATE (
        SUM ( Transactions[ACCTINGCURRAMOUNT] ) * 0.376,
        KEEPFILTERS ( Transactions[REPORTINGCURRENCY] = "USD" )
    )
    + CALCULATE (
        SUM ( Transactions[ACCTINGCURRAMOUNT] ) * 0.475,
        KEEPFILTERS ( Transactions[REPORTINGCURRENCY] = "GBP" )
    )