R Studio Could Not Find "install.packages"

Just a guess: Could there be a problem with the .RData file in your user home directory? I guess uninstall/install RStudio will not remove it (How do I find the .RData file that's making my session unresponsive?). You have to remove it manually.
It is recommended to never store and restore your workspace to/from .RData (First line of every R script?)

If this does not help: Is your installation working except from this error message?

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