Did you try to run library(rpivotTable) immediately after installing rpivotTable or some other package? This sort of error can sometimes happen when you are installing and loading packages in the same session. Generally, it's a good idea to restart R both before and after installing new packages. (Note that's restart R, not RStudio — if you're not sure of the difference, see: Differentiating R from RStudio)
What happens when you restart your R session (Session menu > Restart R) and then run library(rpivotTable) again, before running any other code?