"... Much was" vs "... much was a"

Sentence parsers, including Google's, don't have perfect dictionaries. When faced with a word it does not known, the parser will guess at the word's part of speech. I suspect in this case the parser interpreted the word represented by [this] as an adjective, not a proper name.

The following without the article is correct for a proper name:

Much of the area was Chelsea district.

And the following with the article is correct for an adjective:

Much of the area was a seedy district.

If you capitalize the real word represented above as [this], you may get the opposite advice from the grammar checker.