Why can't the Knight move when there is a Rook next to its two potential movement locations?

There isn't anything about this in "The Basics" part of the tutorial? Is there a rule I don't know about?

For an example of this look at the puzzle lishogi.org/training/HAVEs

the top left knight can't move up for some reason

The knight is pinned by the rook on 73; if it were to move, your king on 78 would be in check.

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