sumif not working

I assume you are looking for a solution in Power Query, applying the transformation before you load the tables into your model.

Table A (same as provided in snip)

Table B (same as provided, plus a client/date, [client c, 3/8/2025], that does not exist in Table A to show how that scenario is handled)

The following two steps (MergeTableA, SumTableASales) pasted into advanced editor will get what you want:

let

    <...>

    PreviousStep = <...>,

    MergeTableA = 
    Table.NestedJoin(
        PreviousStep, {"Client", "Date"}, //change "PreviousStep" to actual step name before this
        #"Table A", {"Client", "Date"}, 
        "Sales", JoinKind.LeftOuter
    ),
    
    SumTableASales = 
    Table.TransformColumns( 
        MergeTableA, 
        { "Sales", each List.Sum( [Sales] )??0, type number } 
    )

in
    SumTableASales

Output: