Why my “baseline composition” is gray?

I want to set the model of biomass pyrolysis, but I found that this “baseline (unreacted)composition” is gray, and the prompt says that it is an solid that needs different densities, I checked that my solid density is different but I still can’t set it, what is going on?
Looking forward to any possible answer, thank you!

The popup text (yellow) is telling you that the inputs for baseline composition are only available when using the variable solids density model, and above you have selected constant density. Try changing that to “Variable”.