Issue with returning a table.

Yes, that's what I meant by code block, thanks, that's much easier to read.

When I copied that code, Power Query complained about the syntax around the [postid=Centreid] section, so I had to mess around with that a bit.

Based on what I understand, I'm wondering if you'd be better off removing the last line so you have a working function that you could then use the 'Add Column' >> 'Invoke Custom Function' option.

So you end up with this...

let
  Centreid = Centreids,
  ids = List.Generate(() => 100000, each _ > 0, each _ - 1),
  url = "https://auth.XXXXXXXX.com/api/v1/auth",
  body = "{ ""user"": ""PCYCNSW-API"",  ""password"": ""XXXXXXX""}",
  tokenResponse = Json.Document(
    Web.Contents(
      url,
      [Headers = [#"Content-Type" = "application/json"], Content = Text.ToBinary(body)]
    )
  ),
  data1 = tokenResponse[data],
  token = "Bearer " & data1[token],
  source = (Centreid as text) =>
    Json.Document(
      Web.Contents(
        "https://office.XXXXXXXXX.com/api/enterprise/booking/center/rooms/summary",
        [
          Query = 10,
          postid = Centreid,
          Headers = [
            #"x-api-key"    = "XXXXXXXXXXXXXXXXXXXXXXXXXXX",
            Authorization   = token,
            #"Content-Type" = "application/json"
          ]
        ]
      )
    )
in
  source

The gif below, Query1 is your code that I pasted above, then add column and invoke the function based on the id column. 

Mine errors obviously because the lack of valid url etc. but hopefully this will get you a little closer.