Using LookupValue to return the Max Value

This works as a calculated column. If you want to use it as a measure, then you need an aggregating function for Account_Data_Total[ERM ID] in your definition of VAR erm.

Try this:

Mapped Account Name =
VAR erm = SELECTEDVALUE ( Account_Data_Total[ERM ID] )
VAR maxqtr =
    CALCULATE (
        MAXX ( Account_Data_Total, Account_Data_Total[Quarter] ),
        FILTER ( Account_Data_Total, Account_Data_Total[ERM ID] = erm )
    )
RETURN
    LOOKUPVALUE (
        Account_Data_Total[Account],
        Account_Data_Total[ERM ID], erm,
        Account_Data_Total[Quarter], maxqtr
    )