How to use FindFit to do kinetic simulation?

I'd like to simulate the chemical kinetic model by fitting the experimental data. Here are my codes for simulation.

data = {{0.`, 0.003207613963640383`}, {0.256348`, 0.751307591324145`}, {0.5004879999999998`, 0.946609520707414`}, {0.7568359999999998`, 0.9997534786329835`}, {0.9887700000000001`, 0.9788588889284163`}, {2.0019530000000003`, 0.7107228417712506`}, {3.00293`, 0.4859859783085131`}, {4.003906`, 0.329989302539268`}, {5.004883`, 0.2236874613858439`}, {6.005858999999999`, 0.15155994391149338`}, {7.006836`, 0.10294001208237556`}, {8.007815`, 0.07060326258953398`}, {9.008785`, 0.048587435830738386`}, {10.009765`, 0.033384617610655296`}};Clear[knn, kf, knf, a];ttime = 10.253905;knf = 0.4;a = 2.36;model = ParametricNDSolveValue[{S1'[t] == (-kf*S1[t] - knn*S1[t]),NN'[t] == (knn*S1[t] - knf*NN[t]),S[t] == a*NN[t],S1[0] == 1, NN[0] == 0}, S, {t, 0, ttime}, {kf, knn}]fit = FindFit[data, model[kf, knn][t], {{kf, 1.5}, {knn, 2}}, t]Plot[model[kf, knn][t] /. fit, {t, 0, ttime},Epilog -> {Red, Point[data]}]

I have tried the condition that kf = 1.5, knn = 2, a = 2.36 can roughly fit the data by using manually change the parameter. However, I get an error like this.

ParametricNDSolveValue:: Unable to find initial conditions that satisfy the residual function within specified tolerances. Try giving initial conditions for both values and derivatives of the functions

I don't know how to fix this problem. I hope if I take {kf, knn, a} these 3 parameters to freely fitting with suitable initial value, I can also correctly fit the data and the rate constants are physically reasonable.(0 < kf, knn < 10, a has no constrained )

I would appreciate any help in rebuilding the code.

Many Thanks!