hi @bragks,
df is a function in the stats package for computing the density of the F-distribution. You want it to be your data frame, but R is calling it as the function. dplyr::select is looking for a data.frame - the error
no applicable method for 'select_' applied to an object of class "function"
is just saying that 'select` doesn't know what to do with a function.
As far as it working in the individual chunk: did you define df as a data.frame in the workspace somewhere outside of your knitr document? If so, R will find it when running the individual chunk with the tiny play button. However, when kniting - R will restart completely. This means Knitr starts with a completely clean environment and your df object is gone. The simple fix is to define df within the body of the knitr document.
Also - I'd suggest changing the name of your data.frame to something more descriptive and try to avoid a name that conflicts with an existing function (like age_df, for example).