It would be helpful to offer a more complete reproducible example of your steps to create and send an email that results in this error. With the info provided, I can't seem to reproduce the error you're getting (and suspect others might be similarly befuddled).
I'm just following the steps here:
https://rdrr.io/cran/blastula/f/vignettes/sending_using_smtp.Rmd
https://rdrr.io/cran/blastula/man/add_attachment.html