I saw this question in an ESL book . Find the mistake and correct it in the following sentence :
"The playground wasn't used by children whom it was built for.
This question came after a brief explanation of relative clauses , so I expected the relative pronoun "whom" to be the mistake but i couldn't see why . Consequently , I thought the mistake could be the missing " the " before the word children . Finally , I said maybe he wanted me to replace" was built " with " had been built " .
😭 please , help me to find out the mistake in the sentence .
Thanks in advance .