Shiny App Error: /v1/applications/ 400 - Validation Error Execution halted

I had the same problem, however, my app name was fine and even adding 'appName =' did not help. Just a side note that this issue came up because I changed the name of my folder in effort to change the name of my app in shinyapp.io

The only thing that worked for me is publishing through the "Publish" button of Rstudio on upper right. I would recommend publishing using that instead of command. You can select files you do not want to publish within the App folder and you can publish the app under a different name then your local name.

Setting an application name solved this problem for me. My application directory contained a space.

deployApp(appName = "myapp")

I had the same error returned. In my case the problem was the name of the app itself. Deployed apps must have names at least 4 characters long with no spaces.