We have successfully demonstrated intensity enhancement of a single high-order harmonic at a wavelength of $46.76\mathrm{nm}$ by using low-ionized tin ions in a laser-ablation plume as the nonlinear medium. The ablation plume was produced by irradiating a solid tin target with a $10\mathrm{mJ}$ energy picosecond laser pulse. Strong 17th-harmonic generation at a wavelength of $46.76\mathrm{nm}$ was observed with a conversion efficiency of about ${10}^{-4}$ . We attribute the strong enhancement of the single high-order harmonic to multiphoton resonance with a strong radiative transition of the $\mathrm{Sn}\mathrm{II}$ ion.