Hi! Welcome!
I’m a little confused about what you’ve been trying to do. Are you trying to deploy a Shiny app or an R Markdown document (or maybe dashboard)?
The error message you report sounds like you might be deploying a Shiny app. If so, it’s probably a good idea to move this post to the #shiny category, so it will be most likely to be seen by people with the right knowledge to be helpful.
It would also be helpful to know where you are deploying — to a server you set up? To a hosted service, like shinyapps.io? (If you are in fact deploying a Shiny app to shinyapps.io, please also add the #shinyappsio tag to your post).
Have you checked the logs to see what went wrong? (Do you know where to find the logs for your deployment setup?)
The problem could be that the mlxR package is not installed on your server (in this case, whether your app runs locally doesn’t matter, because presumably you have all the packages you need installed locally).