The second half of the answer is a design issue. You only have two sequences, so that treatment C only appears in period 3 while treatments A and B only appear in periods 1 and 2. So you have aliasing of treatment with period. So when SAS tries to define an lsmean for C say, it wants to give equal weight 0.333333 to each period, but this is not consistent with assigning weights (0,0,1) to treatment. So your only solution is to remove period from the model. Now there is nothing wrong with your design. In dose escalation studies we regularly alias dose with period. It is done for safety reasons. You always start testing on small doses. Perhaps you have the same thing here with C being a potentially nasty treatment that you want to leave to the end. But, if so, you have to then accept that you cannot correct for period. Rather you build into the execution of the experiment/trial measures to reduce the possibility of such a period effect. James.
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