Issues with Shiny code can stem from both the reactive Shiny code itself or the regular R code used in the functions. In order for us to help you with your question, please provide us a minimal reproducible example (Reprex) where you provide a minimal (dummy) dataset and code that can recreate the issue. One we have that, we can go from there.
I suggest you first try to figure out if the issue is with the R or Shiny. Try and run the R code with some sample data line by line and see if it produces the error (replacing any Shiny variables by regular ones). If not, it might be Shiny related.