In normalising the complete wavefunction of a free partcile; $V(x)=0$
we arrive at
$\int_{-\infty}^{\infty}\Psi_{k} ^\dagger\Psi_{k}dx=|A|^{2}\int_{-\infty}^{\infty}dx=|A|^{2}\left(\infty\right)$
Which implies that this wave function is not normalisable.
Mathematically, it is not normalisable but that's as far as my understanding goes. I would like to see a further and more related explanation for which a non-normalisable wave function implies that a free particle cannot exists in a stationary state and also that such a free particle does not have a definite energy.