It's impossible to permanently miss a Stellazio coin. Locations that cannot be returned to, such as any area on Terra, do not have coins.
However, the thirteenth coin (Ophiuchus) does not become available until the other twelve have been turned in. Ophiuchus is the second coin that appears at Quan's Dwelling (in the same location as Scorpio, actually), so you'll have to find the remaining coins first.
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