Hi, my guess is the issue is not the number of unique values for "stop", but rather the number of stimuli with the same "stop" attribute(s) defined in that trial template. Because you are asking FindingFive to present stimuli that repeat the "stop" attribute *at most* once in a row (the max_N setting), it's not possible to find a sequence that satisfies this constraint given your setup.
Does that seem true to you?
Does that seem true to you?