It is challenging to debug shiny apps, and you need to pay very close attention to the structure of the values that are being supplied by the various input$___ elements — it’s not necessarily the same as placeholder values you put in yourself outside of shiny.
These resources are worth investing some time in studying:
It would certainly be useful to see if the same error occurs on somebody else’s system, but unfortunately your example above still isn’t quite self-contained. The first stumbling block I see: the app code is source-ing two scripts you haven’t provided (and which don’t seem relevant here).
Can you please check the example carefully to make sure it really will run elsewhere? One way to do that would be to create a new RStudio Project for a new shiny app with just this code in it and see if it runs on your system.
You might also take a look at the advice here: Shiny debugging and reprex guide