PQ: How to split a large column into 50 characters columns but split only at whitespace?

DanHuber 

is there a specific reason why your file is using an external data connection?

Nope. That's the way it is for a moment with 365. TBH I never tried to figure out the reason

Query code is, assuming data formatted as table named Table, with your data in [Column1]:

let
    Source = Excel.CurrentWorkbook(){[Name="Table"]}[Content],

    PartOne = Table.AddColumn(Source, "Part1", each
        let
            range = try Text.Range([Column1], 0, 50)
        in
            if range[HasError] then [Column1]
            else if Text.At(range[Value], 49) =  " " then range[Value]
            else Text.BeforeDelimiter(range[Value], " ", {0,RelativePosition.FromEnd}) & " ",
        type text
    ),
    PartTwo = Table.AddColumn(PartOne, "Part2", each
        let
            after = Text.AfterDelimiter([Column1], [Part1]),
            range = try Text.Range(after, 0, 50)
        in
            if range[HasError] then after
            else if Text.At(range[Value], 49) =  " " then range[Value]
            else Text.BeforeDelimiter(range[Value], " ", {0,RelativePosition.FromEnd}) & " ",
        type text
    ),
    PartThree = Table.AddColumn(PartTwo, "Part3", each
        Text.AfterDelimiter([Column1], [Part1] & [Part2])
    ),
    KeptParts = Table.SelectColumns(PartThree, {"Part1", "Part2", "Part3"}),
    FixedNullLength = Table.ReplaceValue(KeptParts, each [Part1], null,
        (x,y,z) as nullable text => if Text.Length(x) = 0 then null else x,
        {"Part2", "Part3"}
    )
in
    FixedNullLength

NB: Last step (FixedNullLength) isn't necessary if you don't mind having non-null cells ("") in the output