So I'm writing a narrative for LA class, and there's a spot in my introduction that, in my opinion, should have a comma, but returns a grammar error by MS Word whenever I try it - hence why I am a little confused as to what to put there. When I put a semicolon, it does not return an error, but the semicolon does not separate two distinct clauses, so it's incorrect as far as I can tell. Here's the relevant part of my introduction (italicised):
In an alternate life, Mark would’ve lived a life considered by many to be somewhat normal. He could’ve been president of his country, the United States of America. He could’ve been a millionaire, a businessman, a scholar, a politician, a doctor, a scientist, anything… but here he was<,> an American infantry soldier of 1916, stuck in a dirty, bloody hole.
Note: the incorrect punctuation is the <,> (the symbols are only there to show where the comma is), and the two parts of the sentence that need separation are emboldened.
Thank you for your help.