Mapping in R-Studio

I am trying to create a pH map on R, but once I did the summary of the prediction model, it gives :NULL for class and mode. Which results in my raster not showing the pH variability. It appears as I had only one class. Could somebody here help me solve this , please?

 **names(predicao)
[1] "fit""lwr""upr""x""y"> summary(predicao$model)
Length  Class   Mode 
     0   NULL   NULL**