less capable than desirable (idiom); not as strong as one would wish
the spirit is willing but the flesh is weak
Definitions (粵典–words.hk)
語句
能力不足以達成心願、抱負 to be unable to do as well as one's wish or ambition
Definitions (Wiktionary)
phrase, idiomatic
to lack the ability to do what one wants; one's strength does not match one's ambitions; the spirit is willing but the flesh is weak (syn.) 心余力绌, 心餘力絀, 心有餘而力不足, 心有余而力不足