I cannot open a source someone cited.

1. Try the source directly rather than through the citation. Public sources move, and a citation records where something was when it was read.
2. If it has moved or gone, say so on the question. A source that cannot be reached is a fact about the question and worth recording, not a private inconvenience.

Some evidence is visible only to the desk that recorded it. If you cannot see it at all, it was not published to you, and that is a boundary rather than a fault.