[noun] + の + うちに入らない [verb (plain)] + うちに入らない [i-adj] + うちに入らない [na-adj] + な + うちに入らない
うちに入らない says something does not really count as what it seems to be: "that's not hardship", "that's not really exercise". A noun takes の in front; a verb or adjective goes in its plain form, a な-adjective with な. What is being judged usually comes first with は.
Often the noun is the same word twice: the thing, and the category it fails to reach. ていど程度 and ぐらい in front mark how small it is.
Said about yourself, it is modesty: you play a little, but that is not really "playing the piano". Said about someone else or what they went through, it belittles, and it can sound harsh.