Is it right to say every sentence can be an utterance but not every utterance can be a sentence?

Yes, every sentence can be an utterance but not every utterance can be a sentence. This is because an utterance can be just an action of saying or expressing something aloud which may not be a sentence. However, every sentence (that is spoken) can be an utterance because sentences can be said to be a more 'complete' version of an utterance.