Take a look at the sentence again:
Derek wanted a ferocious pit bull terrier to scare off
burglars, but his apartment complex did
not allow dogs as a result, Derek had to settle for naming his goldfish
Mr. Jaws and installing additional door locks.
Derek wanted a ferocious pit bull terrier to scare off burglars is a main clause. His apartment complex did not allow dogs is also a main clause. A comma and a coordinating conjunction like but can connect two main clauses.
Go back to the sentence to try again.