This is just my theory, but maybe, all the martial arts he copies from martial art novels require inner energy or chi, which also can be said mana, thus he can't use the skill and it became normal kick and punch... Since his stats is S grade, the attack become powerful but it doesn't mean he can use skill which requires mana...
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Not certain but isn’t it that mana strengthens his abilities? Like what was said before he can’t defeat that first climber if he dosn’t learn mana, and I’m assuming this new climber is stronger than the ones before so defeating him without mana would be even more improbable
He has only just learnt how to use mana and some powerful moves, but for the entire time before that he has been able to fight easily using just his strength that's why all his other stats are at S rank. Why is it now that not being able to use mana has basically crippled him??? feeling like the author is lacking some consistency in the MC's abilities （￣へ￣）