One mole of a monoatomic real gas satisfied the equation p(V - b) =...

$P\left(V - b\right) = RT$
$\to PV - Pb = RT$
$\to \dfrac{PV}{RT} = \dfrac{Pb}{RT} + 1$
$\to Z = 1 + \dfrac{Pb}{RT}$
Hence, $Z > 1$ at all pressures.
This means, repulsive tendencies will be dominant when interatomic distance are small. This means, interatomic potential is never negative but becomes positive at small interatomic distances.