It's from:
On my local machine (Ubuntu) i fixed this by installing the mosquito server thus:
sudo apt install libmosquitto-dev libmosquittopp-dev
Not sure if I can accomplish the same on shinyapps.io
Per the documentation on using packages on shinyapps.io, if a system package is not available by default, please open an issue or pull request in the shinyapps-package-dependencies GitHub repo.