Preparation for lessons

Find some mistakes in English! What's the difference between не and нет?

Preparation for a Russian language lesson. Level B1 subtest "Grammar. Vocabulary". Let's start with exercise 63.
Сергей встретил ... около метро.
Потом Сергей вместе ... вернулся
домой.

There are mistakes in one sentence. Can you find them? Write me a message about it and the lesson will be 10 minutes longer.

I can send you a link to the training materials.