You're missing Authorization header with valid token in GetModelAssets function:

url = baseUrl & "/modeldata/" & urn & "/scan",

input = "{""families"": [ ""n"", ""z"" ], ""includeHistory"": false, ""skipArrays"":true}",
response = Web.Contents(
  url,
  [
    Headers = [#"Authorization" = "Bearer " & accessToken, #"Content-Type"="application/json"],
    Content = Text.ToBinary(input)
  ]
),

Every call to the Tandem REST API needs to be authenticated - see here.