"He needs more practice."
Translation:Han behöver mer övning.
Yes, that's ungrammatical in much the same way as English "he needs practice to" would be. But you could say "han behöver lite övning till" and it would mean "he needs a little more practice".
fler is for countable things. You could say fler övningar, but then that would mean 'he needs more exercises' (like, a larger number of maths exercises or something like that).
mer is for uncountable things or things viewed as a mass, and fler is for countable things.
Careful Duo, this sounds like a dig... I'm not saying it's untrue, mind you
For me it's quite interesting whether it is possible to find something similar in English/German to the word "övning"? That would allow to memorize this word with much greater simplicity. Any ideas?