Error: Unable to compute predicted values with this model. You can try to
  supply a different dataset to the `newdata` argument. This error was
  also raised:
  
  there is no package called 'collapse'
  
  Bug Tracker:
  https://github.com/vincentarelbundock/marginaleffects/issues

model <- stan_glm(Sepal.Width ~ Species, data = iris, refresh = 0)
contrasts <- modelbased::estimate_contrasts(model, contrast = "Species")
means <- modelbased::estimate_means(model, by = "Species")