Review the original item:
Sandra spent every dime on her vacation to Cancun, Mexico, where she went
shopping, dancing, swimming, and sightseeing,
she does not care that her rent will be late or that she will have
to eat macaroni and cheese for a month.
You wanted to fix it this way:
Sandra spent every dime on her vacation to Cancun, Mexico, where she went
shopping, dancing, swimming, and sightseeing she
does not care that her rent will be late or that she will have to eat macaroni
and cheese for a month.
To delete the comma between sightseeing and she would cause an equally bad problem, a fused sentence. A fused sentence occurs when you have two complete sentences joined with no punctuation whatsoever. Sandra spent every dime on her vacation ... begins the first sentence. She does not care ... starts the second sentence.
The spot between sightseeing and she needs a stronger break than blank space.
You might want to consult the rules for fixing comma splices and fused sentences.