The following three methods work for me to mark blocks of code as comments in the code pane of R Studio.
While in insert mode, use Shift + Arrow to select text then Shift + Control + C to make every line that has text selected on it a comment.
Starting from normal mode, press Shift + V to enter visual mode with line selection. Use the arrows to select lines then Shift + Control + C to mark the lines as comments.
In normal mode, use Shift + Arrow to select text then Shift + Control + C to make every line that has text selected on it a comment.
I wrote my response to show that Shift + Control + C does work on my system, thinking that it could be made to work on your system.
In any case, I can use Shift + V and then the arrow keys to select lines. Typing : then activates command mode and the command line shows '<,'> . If I type s/^/#/ and press Enter, all of the selected lines become comments.