Hi, and welcome!
Please see the FAQ: What's a reproducible example (`reprex`) and how do I do one? Using a reprex, complete with representative data will attract quicker and more answers.
The error is fairly clear: wherever ONS_data.Rdata is, it's not in the working directory. But how to fix it isn't without knowing where in the program the message comes from. So that's the first step. (I'm not a bright shiny object myself, so for all I know it may not even be possible to work with .Rdata objects in shiny, but won't be able to find out without more to go on.)