The model converges if the response variable is log(Mn).   However, I will take @StatsMan's comment a bit further and suggest that your design (only six subjects for a six-period, six-treatment Latin square design) may be (and probably is) much too small to be useful or reliable. I would be concerned about balance and carryover, especially because of a period effect (Mn decreases over periods). A model that converges is necessary but may not be sufficient. It is convenient but treacherous to underestimate the complexity and needs of Latin square/crossover designs.   I hope this helps.   ... View more