how to replace all special character with space

You need to have an approach where you need to replace all other characters other than allowed. In below approach, a to z and 0 to 9 are allowed. Hence, it will replace any other character encountered

{"A".."Z","a".."z","0".."9"} - If you feel 0 to 9 not allowed, make it {"A".."Z","a".."z"}

So basically you are taking Whitelist approach rather than Blacklist approach as creating Blacklist is a big task and also prone to error.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45W8sjU89TPyS9Lja/ML1WK1YlW8q10yEvMTVXNLAZzwzNSi1JjSg0MjMwSi1K1gaoU0/Mz89LBko4pKUWpxcUKmcUKxiY6Cs75OTmZeUB9sQA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Column1 = _t]),
    BuffListAllowedChars = List.Buffer({"A".."Z","a".."z","0".."9"}),
    SourceList = List.Buffer(Source[Column1]),
    ListCount = List.Count(SourceList),
    GenList = List.Generate(()=>[x=Text.Combine(List.Transform(Text.ToList(SourceList{0}),(a)=>if List.Contains(BuffListAllowedChars,a) then a else " ")),i=0], each [i]<ListCount, each [i=[i]+1, x=Text.Combine(List.Transform(Text.ToList(SourceList{i}),(a)=>if List.Contains(BuffListAllowedChars,a) then a else " "))], each [x]),
    Result = Table.FromColumns({GenList},{"Result"}) 
in
    Result