Get shiny to produce an error message and not upload file


I have created an app that allows users to upload pdf files to Rstudio connect and the uploaded file gets tags according to the user inputs provided.

The way the app currently works is that the user provides input in input boxes and these inputs are used to tag the file in the correct place. Is there a way I can create an error message if the incorrect/invalid inputs are given?

Example: If a person types in Mach instead of March, can the app give an error?
What currently happens is that R gives an error the 'Tag not found' but the file gets upload to Rstudio connect but it doesn't get tagged. Is there a way to stop a file from getting uploaded if the tag schema is not found?

Thank you

Yes, the upload controller could be disabled or hidden if other inputs are not correct, and it can be enabled or shown when they are correct.
shinyjs might be useful, or just uiOutput/renderUI could do such things.