Take a look at the original sentence again:
Sandra spent every dime on her vacation to Cancun, Mexico, where she went
shopping, dancing, swimming, and sightseeing, she
doesn't care that her rent will be late and that she will have to eat macaroni
and cheese for a week.
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
doesn't care that her rent will be late and that she will have to eat macaroni
and cheese for a week.
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 doesn't 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.
Go back to the sentence to try again.