Dash R Tutorial Isn't Working - R Terminal "source is not recognized..."

Thank you for reading! Please may someone help me get set-up in Dash?
This is almost surely a dumb question, but I have no experience with the command line or computer science education. I’m in Windows 10, it’s fully updated

In part 2 of the R-Dash tutorial it says:

Note: Throughout this documentation, R |julia Julia code examples are meant to be saved as files and executed either using source("app.R") in your R terminal or using Rscript app.R in a shell. Throughout this documentation we’ll use the former method.

I’ve saved the R script as “app.R” in my working directory.
I’ve tried using the R terminal using source(“app.R”) so my command looks like this:


I receive the following error:

How do I get this to work? I have no idea what a shell is.

Again - sorry for this basic stuff. If there are resources I can read to understand better please let me know! There’s a lot of unknown unknowns at the moment.

Thank you!