o espírito está pronto, mas a carne é fraca

心は意欲的だが肉体が弱い（弱さのせいで何かを達成することは不可能だ）

English Meaning

the spirit is willing but the flesh is weak (achieving something is made impossible by frailty)

彼はその献身にもかかわらず、最終的にいくつかの状況では、心は意欲的だが、肉体は弱いことに気づいた。

Despite his dedication, he eventually realized that, in certain situations, the spirit is willing but the flesh is weak.