The second argument of PropagatorDenominator should not contain I.
So either
ToFI[Expand[(32*CA^3*mE^2*FeynAmpDenominator[PropagatorDenominator[Momentum[p1, D]],
PropagatorDenominator[Momentum[p2, D], mE], PropagatorDenominator[
-Momentum[p1, D] + Momentum[p2, D], mE], PropagatorDenominator[
-Momentum[p1, D] + Momentum[p2, D], mE], PropagatorDenominator[
-Momentum[p1, D] + Momentum[p2, D], mE]]*Pair[Momentum[p1, D], Momentum[q, D]]*
Pair[Momentum[p2, D], Momentum[q, D]]*((-Pair[LorentzIndex[Lor1, D], Momentum[q, D]])*
Pair[LorentzIndex[Lor2, D], Momentum[q, D]] +
Pair[LorentzIndex[Lor1, D], LorentzIndex[Lor2, D]]*Pair[Momentum[q, D], Momentum[q, D]])*
SMP["g_s"]^6*Subscript[zeta, 1]*SUNDelta[SUNIndex[a], SUNIndex[b]])/
((-1 + D)*Pair[Momentum[q, D], Momentum[q, D]])], {p1, p2}, {q}]
or, shorter,
ToFI[(32*CA^3*mE^2*FAD[p1, {p2, mE}, {-p1 + p2, mE}, {-p1 + p2, mE}, {-p1 + p2, mE}]*SD[a, b]*
SMP["g_s"]^6*SPD[p1, q]*SPD[p2, q]*(-(FVD[q, Lor1]*FVD[q, Lor2]) + MTD[Lor1, Lor2]*SPD[q, q])*
Subscript[zeta, 1])/((-1 + D)*SPD[q, q]), {p1, p2}, {q}]
results in
(32*CA^3*mE^2*SD[a, b]*SMP["g_s"]^6*(-(FVD[q, Lor1]*FVD[q, Lor2]) + MTD[Lor1, Lor2]*SPD[q, q])*
Subscript[zeta, 1]*TFI[D, SPD[q, q], {0, 0, 1, 1, 0}, {{1, 0}, {1, mE}, {0, 0}, {0, 0},
{3, mE}}])/((-1 + D)*SPD[q, q])
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