why if G is an abelian p-group not divisible then exists an element g in G which is not divisible by p? thanks
As Pace said, but with more detail:
If $G$ is an abelian $p$-group, then for any $g$ in $G$, the order of $g$ is a power of $p$, say $p^k$. Thus for any integer $n$ coprime with $p$, $n$ is a unit (mod $p^k$), so for some $m$, $nm=1$ mod $p^k$. So $n(mg)=(nm)g=(ap^k+1)g=g+a(p^kg)=g+0=g$. Thus $g$ is divisible by $n$.
This holds for any $g$; so if every $g$ is divisible by $p$, they are also divisible by $p^n$ for all $n$, so they are all divisible by $p^nk$ for any $n$ and any $k$ coprime to $n$, which is to say, any nonzero number.