Why does Caret featurePlot returns returns NULL?

I am trying to use featurePlot of caret in R.

This is the code I am using:

trellis.par.set(theme = col.whitebg(), warn = FALSE)
caret::featurePlot(x = df[, 3:6],
y = df$ud, plot = "scatter")  where all the columns of df[, 3:6]are of numeric type, while df$ud is a binary factor.

I also have installed the library ellipse as suggested elsewhere.

However, I keep getting NULL.

Any idea why?

• Take a look at help(featurePlot) in caret for the expected class of the argument, x, which is a matrix – Richard Careaga Nov 7 '19 at 5:48

typeof(df\$ud)