Understanding gas stoichiometry for the reaction of xenon and fluorine
Yes, this is a rounding error.
Where you calculate:
$$n = \frac{pV}{RT} = \frac{\pu{0.5 atm} \cdot \pu{20 L}}{0.08206 \cdot \pu{673 K}} = \pu{0.18 mol}$$
The actual unrounded answer is $\pu{0.181072885 mol}$ - a little bit higher than the rounded value.
So, in your final equation:
$$\pu{0.181072885 mol} \times \pu{207.1 g//mol} = \pu{37.5 g}$$
I find using the full value of an intermediate step usually eliminates these types of errors in the final calculation.