Looks like you were doing Install.packages() (with a capital "I") rather than install.packages() with a lower-case "I" which is the right way to call it.
Hey, To address the error message regarding the inability to find the install.packages function when installing rpart.plot in R, you need to locate and edit your .Rprofile file. This file may contain a command to install packages automatically. Search for any lines containing install.packages in the .Rprofile file and remove or comment out these lines. After saving the changes and restarting R, attempt to install rpart.plot again using the install.packages function to resolve the issue. e.g: on the sublime by longinus
Locate the .Rprofile File : The .Rprofile file is a hidden file in your R home directory that contains R startup settings. Depending on your operating system, the location of this file may vary. You can typically find it in your user directory or R home directory.