Here is great in rstudio project as it helps build path. For shiny apps to deploy, you generally have a more somple organisation. The data folder should be at the root of you app directory, and the relative path should be enough.
donnees <- read.xlsx("data/donnees.xlsx"), sheetName = "donnees", encoding = "UTF-8")
In fact it is possible that here is not well adapt to shiny apps deploy in shinyapps.io. Or at least, check the discovery logic of here
as you may need a .here file in the root directory to help find the path.
Also when deploying you need to check that the file is included in the published bundle. You should see that in the IDE when deploying or in the bundle manifest that you should find on the website dashboard. Check that the xlsx file is there.