Learn Russian: Эту ошибку нельзя скрыть, её увидят все. - This error cannot be hidden, everyone will see it.

This is the core difference between aspects when using нельзя. When you use нельзя with a perfective verb like скрыть, it means it is physically or logically impossible to achieve the result (impossible to successfully hide). If you used the imperfective скрывать, it would mean it is prohibited or a bad idea to do the action (you must not hide it). Here, the error is so obvious that hiding it simply cannot be done.

Why is Эту ошибку in the accusative case when the English translation says 'This error cannot be...'?

In English, 'This error' is the subject of a passive sentence. But the Russian sentence is active and impersonal: literally, 'It is impossible to hide this error.' Эту ошибку is the direct object receiving the action of the verb скрыть (to hide), so it takes the accusative case. It is placed at the front of the sentence to emphasize it as the main topic.

Why is the pronoun её used to mean 'it' in the second half of the sentence?

Её literally translates to 'her'. In Russian, grammatical gender applies to inanimate objects too. The word for error, ошибка, is feminine. When replacing it with a pronoun in the accusative case (since it is the object that everyone will see), you must use the feminine form её.

The second part is её увидят все. Is все the subject here even though it is at the end?