Simply stated, 'mustn't' is a difficult word to say. It is not a word that Americans say often. I, personally, think the word looks odd when written. Because of this, I would not use the contraction for must not. I would say, 'Eric must not like pickles.'
"He can't like pickles' is incorrect. Is he incapable of liking pickles? Or, is it a matter of simply not liking the taste of pickles?
We eat everything 'on' our plates because a plate does not have an inside section. But, we can eat everything 'in' our lunch box because a lunch box does have an inside section under the cover. Eric ate everything in his lunch box.
Does this answer your question?
Student or Learner