Generating pair higgs with decaying

Dear experts,

My goal is to generate the collision events data of pair higgs production with both higgs decaying into bb~ under standard model.

Im trying to generate the events of p p > h h [QCD] with the decay using Madspin to decay the higgs into 4b. Then, do the showering and detection simlulation using Pythia and Delphes. However, after getting the cross section of this process, There is error in Madspin level. Im wondering how can I fix this this problem under loop_sm model. The following is my command and the error message.

import model loop_sm
generate p p > h h[QCD]
output /data/Chen-Wang/gghhtest
launch
shower=Pythia8
detector=Delphes
madspin=ON

### madspin Card ###

#Some options (uncomment to apply)
#
# set seed 1
# set Nevents_for_max_weight 75 # number of events for the estimate of the max. weight
# set BW_cut 15 # cut on how far the particle can be off-shell
# set spinmode onshell # Use one of the madspin special mode
 set max_weight_ps_point 400 # number of PS to estimate the maximum for each event

# specify the decay for the final state particles
decay t > w+ b, w+ > all all
decay t~ > w- b~, w- > all all
decay w+ > all all
decay w- > all all
decay z > all all
decay h > b b~
# running the actual code
launch

######

no any changes of run card, parameter card. The following is the error message in the MadSpin level:

INFO: generating the production square matrix element
INFO: generate p p > h h --no_warning=duplicate;define pert_QCD = -4 -3 -2 -1 1 2 3 4 21;add process p p > h h pert_QCD --no_warning=duplicate;
Command "generate_events run_01" interrupted with error:
NoDiagramException : No amplitudes generated from process Process: g/u/c/d/s/u~/c~/d~/s~ g/u/c/d/s/u~/c~/d~/s~ > h h WEIGHTED=8 @1. Please enter a valid process
quit

Let me thank you for the answering first.

Best
Wang