Warning in install.packages : cannot remove prior installation of package ‘data.table’ [duplicate]

Answer 1

I assume you're on Windows. You may have data.table loaded into your session already; on Windows, dlls that are part of a package can't be unloaded until R shuts down. So restart R and then try running install.packages again.

Answer 2

Try running R with admin access (sudo in Linux, or for Windows, right click and choose "Run as Administrator"). I had the same problem when running R from PowerShell in Windows, and this fixed it.