How do I add a column to an R dataframe?

Question

I would like to add a column to an existing R dataframe. When I tried using the mutate() function, it seemed to work without error:

mutate(Bikeshare_202101, Month=1)

The new column can be seen in the screen dump. When I check with the str() function, however, the new column does not appear:

str(Bikeshare_202101)

I would have attached some cropped PNG images of RStudio to help explain, but I am not allowed to do so yet. Thanks, John