Adding Custom Column To Obtain Prior Month Balance

AlienSx, I don't understand where, when, or how you're grouping to get the results.

This is the Power Query with your code embedded; presumably I did this correctly:

let
Source = #"Trial Balance",
#"Merged Queries" = Table.NestedJoin(Source, {"Account Number"}, #"Account Category", {"Account Number"}, "Account Category", JoinKind.LeftOuter),
#"Expanded Account Category" = Table.ExpandTableColumn(#"Merged Queries", "Account Category", {"Statement"}, {"Account Category.Statement"}),
#"Filtered Rows" = Table.SelectRows(#"Expanded Account Category", each ([Account Category.Statement] = "Income Statement")),
f = (tbl as table) as table =>
[sorted = Table.Sort(tbl, "Date"),
prior_month = {0} & List.RemoveLastN( tbl[Balance], 1),
out = Table.FromColumns( Table.ToColumns(tbl) & {prior_month}, Table.ColumnNames(tbl) & {"Prior Month"})] [out],
gr = Table.Group(#"Filtered Rows", {"Entity", "Account Number", "Description", "Account # - Description" }, {{"all", each f(_)}}),
expand = Table.ExpandTableColumn(gr, "all", {"Date", "Balance", "Prior Month"}, {"Date", "Balance", "Prior Month"}),
#"Changed Type" = Table.TransformColumnTypes(expand,{{"Balance", Currency.Type}, {"Prior Month", Currency.Type}, {"Entity", type text}, {"Date", type date}})
in
#"Changed Type"

Below is the default output after the query. Note the prior month values equate to the previous row values , but the previous row isn't necessarily the previous month:

This is what it looks like after sorting by Date: