mutate() does not have a . after it.
However, I think you are getting your objects mixed up. Whilst you can operate on a data.table object using mutate() and other tidyverse functions, I'm not sure why you want to mix the methods.
I'd either use tibbles (or data frames) with tidyverse functions, stick with data.table syntax or use dtplyr to mix the two if you know what you are doing.