Take a look at the original sentence again:
Nathalia likes to get good grades but hates when her friends call her a
nerd, sometimes Nathalia will claim that
she got a 77 on a test when, in reality, she earned a 97.
You wanted to fix it this way:
Nathalia likes to get good grades but hates when her friends call her a
nerd sometimes Nathalia will claim that
she got a 77 on a test when, in reality, she earned a 97.
To delete the comma between nerd and sometimes would cause an equally bad problem, a fused sentence. A fused sentence occurs when you have two main clauses joined with no punctuation whatsoever. Nathalia likes to get good grades is the first main clause. Sometimes Nathalia will claim is the second main clause. The spot between nerd and sometimes 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.