The error message "These values cannot be compared" is misleading unfortunately. The real reason, as Mike and Alan already pointed out, is, that getting the extrema by setting the first derivative to zero will only work for well behaved continuous functions. You had luck yesterday and bad luck with the new function today, as the derivative seems to never get zero and even if it would it probably wouldn't be the absolute maximum.
Yesterday I wrote "I guess your solve block may be more versatile and probably more robust when we come across more "ill-natured" functions". Thats proves true today, even though I should have written maximize instead of solve block (you can delete the "Given" - you can use maximize as a standalone function.