PropagatorDenominator[Momentum[q], m] is a factor of the
denominator of a propagator. If q is supposed to be D-dimensional, use
PropagatorDenominator[Momentum[q, D], m]. What is meant is
1/(q^2-m^2).
PropagatorDenominator must appear inside
FeynAmpDenominator, it is not a standalone object.
Overview, FeynAmpDenominator, FeynAmpDenominatorExplicit.
FeynAmpDenominator[PropagatorDenominator[Momentum[p], m]]\frac{1}{\overline{p}^2-m^2}
FeynAmpDenominator[PropagatorDenominator[Momentum[p, D], m]]\frac{1}{p^2-m^2}