Hey there. Master difficulty is only available as part of the weekly replayable mission. Those shouldn't normally (?) be accessible until you've completed the campaign on 'Classic' or 'Legendary' modes, did someone else launch it and you joined?
Either way, that kind of completion won't progress the campaign. It has to be done on Classic or Legendary, launched from your screen, whatever mission node you can see on the map.
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Thank you. Someone else did laugh it and I joined. I assumed, and that seems to cause issues, that it would count. Guess I will have to try again. Thank you for the clarification
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No problem!
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Launch it that is