You cannot do it solo or in preservation mission. Bungie patched it so that it now requires 4 players doing each of the symbols.
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Then why leave this opportunity in a mission that can be completed solo?
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Probably because that part is just copy/pasted from the raid coding.
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So now each player has to take his own sign?
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Correct. 4 different players need to do 1 symbol/action each.