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I tried to install package in R studio Version 4.1.0. but it says "
A version of this package for your version of R might be available elsewhere,
see the ideas at
R Installation and Administration"

I use Windows 10 OS. Here is the screenshot of the message.

The last package you where trying to install in that screenshot is actually called ggplot2 not ggplot that is why it says it os not available.

And this is not related to not having RTools installed in your system, which by the way, it would be a good idea since you would need it to install packages from source when there are no binaries available.

If I do install.packages(ggplot2)
It throws
Error in install.packages : object 'ggplot2' not found

How would I know if the source has no binaries?

You have to use quotes, like you did in the screenshot

install.packages("ggplot2")

The console output will let you know