Is there a word/ phrase to describe somebody who has devoted their life to practising something but is still not very good at it?

I'd suggest Manqué, a somewhat uncommon term borrowed from French. Wikipedia describes it thus:

Manqué (feminine, manquée) is a term used in reference to a person who has failed to live up to a specific expectation or ambition. It is usually used in combination with a profession: for example, a career civil servant with political prowess who nonetheless never attained political office might be described as a "politician manqué". It can also be used relative to a specific role model; a second-rate method actor might be referred to as a "Marlon Brando manqué".

Frustrated.

(of a person) unable to follow or be successful in a particular career. "a frustrated actor" - dictionary.com

My father always called himself a "frustrated musician". He was a physicist and loved music, but in his opinion, his love for it exceeded his talent. Though, to be fair, he was the first person who could get a sound out of my didgeridoo the first time he held it to his lips.

This is likely to require a lookup, but it's an easy one and the meaning (and connotation!) are unambiguous: a Florence Foster Jenkins.

Wikipedia has a good overview; she was someone who wanted to be an opera singer since she was a child, rich enough that nobody was willing to tell her that she wasn't any good, and determined enough that at age 76 she rented Carnegie Hall for her only public performance. She's been described as "the world's worst opera singer".

A life time practising and he's just a flop.

Oxford Definition :

Informal (of a performer or show) be completely unsuccessful; fail totally. ‘the show flopped in London’

It might be best not to overthink it. A talent is a person's natural aptitude or ability for something, so someone who lacks this ability is talentless.