  In the first part of this paper, we consider the problem of fill-in of
nonnegative scalar curvature (NNSC) metrics for a triple of Bartnik data
$(\Sigma,\gamma,H)$. We prove that given a metric $\gamma$ on
$\mathbf{S}^{n-1}$ ($3\leq n\leq 7$), $(\mathbf{S}^{n-1},\gamma,H)$ admits no
fill-in of NNSC metrics provided the prescribed mean curvature $H$ is large
enough (Theorem \ref{Thm: no fillin nonnegative scalar 2}). Moreover, we prove
that if $\gamma$ is a positive scalar curvature (PSC) metric isotopic to the
standard metric on $\mathbf{S}^{n-1}$, then the much weaker condition that the
total mean curvature $\int_{\mathbf S^{n-1}}H\,\mathrm d\mu_\gamma$ is large
enough rules out NNSC fill-ins, giving an partially affirmative answer to a
conjecture by Gromov (see P.\,23 in \cite{Gromov4}). In the second part of this
paper, we investigate the $\theta$-invariant of Bartnik data and obtain some
sufficient conditions for the existence of PSC fill-ins.
