Let M be a complete non-compact Riemannian manifold and \(\sigma \) be a Radon measure on M. We study the existence and non-existence of positive solutions to a nonlocal elliptic inequality \(\begin{aligned} (-\Delta )^{\alpha } u\ge u^{q}\sigma \quad \text {in}\,\,M, \end{aligned}\) with \(q>1\) . When the Green function \(G^{(\alpha )}\) of the fractional Laplacian \((-\Delta )^{\alpha }\) exists and satisfies the quasi-metric property, we obtain necessary and sufficient criteria for existence of positive solutions. In particular, explicit conditions in terms of volume growth and the growth of \(\sigma \) are given, when M admits Li-Yau Gaussian type heat kernel estimates.