List Sum except specific value

Your basic idea is correct. The below works:

List.Sum(Table.SelectRows(#"Changed Type", each [Category] <> "Cat 8")[Location 1])

You need to include the field you want to sum at the end. Appending [Location 1] tells it what column of the filtered table to sum.

Full code:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("XY8xDsIwDEWvUmWuLNtJbGdm5QZVhwohxEIl1IXbE6coFDbnvfzvZJrCadmut/X5CmM4r5dlu6+PgY4HHsI8tnuNZ9IMUgdSzsDdcUUFMTUXEQlyjN1Gh6S2JzGXH5u8Vyy7s5I7d6AJEawOnESgHFJ7FVqzMQqwiHarnv1I/pcOSUwMuK1mBj00lwotJXdslL7/xyZwF/VdrXN+Aw==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"(blank)" = _t, #"(blank).1" = _t, #"(blank).2" = _t]),
    #"Promoted Headers" = Table.PromoteHeaders(Source, [PromoteAllScalars=true]),
    #"Changed Type" = Table.TransformColumnTypes(#"Promoted Headers",{{"Location 1", Currency.Type}, {"Location 2 ", Currency.Type}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Custom", each List.Sum(Table.SelectRows(#"Changed Type", each [Category] <> "Cat 8")[Location 1]))
in
    #"Added Custom"

 How to use M code provided in a blank query:
1) In Power Query, select New Source, then Blank Query
2) On the Home ribbon, select "Advanced Editor" button
3) Remove everything you see, then paste the M code I've given you in that box.
4) Press Done
5) See this article if you need help using this M code in your model.