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, but sometimes Nathalia will claim
that she got a 77 on a test when, in reality, she earned a 97.
A comma and a coordinating conjunction like but is a perfectly good way to join two main clauses. There is a catch, however. The coordinating conjunction that you choose must logically join the two main clauses. In the example above, the conjunction but, which shows contrast, is inappropriate.
You might want to consult the rules for fixing comma splices and fused sentences.
Go back to the sentence to try again.