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.
As a result is an introductory transition. Derek had to settle for naming his goldfish Mr. Jaws is a main clause. You can connect an introductory transition to a main clause with a comma.
Go back to the sentence to try again.