  In this paper, we consider the low-energy scale inverse seesaw mechanism in
which the observed neutrino mass and lepton mixing are explained by introducing
right handed neutrinos and the gauge-singlet fermions with experimentally
testable energy scale. Moreover, the presence of such new fermions leads to
unitarity violation in lepton mixing due to significantly large mixing between
active neutrinos and the heavy fermions. In addition to this, such large lepton
mixing also gives rise to potentially large lepton flavor violation, which
allows to constrain the non-unitarity parameters via lepton flavor violating
decays ($l_i \to l_j \gamma$). We make use of these constraints on
non-unitarity parameters and investigate their effects on the determination of
current unknown oscillation parameters at long-baseline experiments. We find
that non-unitarity parameters are sensitive to NO$\nu$A experiment. However, it
is observed that NO$\nu$A experiment is not expected to improve the current
knowledge of non-unitarity parameter $\eta_{21}$. We also find that the
sensitivities to current unknowns are deteriorated significantly in presence of
non-unitary lepton mixing and these sensitivities crucially depend upon the new
CP-violating phase in the non-unitary mixing. Further, we find that the
degeneracy resolution capability of NO$\nu$A experiment is reduced in the
presence of non-unitarity parameters. However, the synergy between the
currently running experiments T2K and NO$\nu$A can improve the parameter
degeneracy resolution and hence there is enhancement in the sensitivities of
unknowns.
