I meant no offence, it wasn't clear that you were a desktop user acting as their own 'admin'. Many users of R are not. I have experience as an R user on windows desktop, as well as my companies hosted R server, on a commercial linux platform. As such I have an Admin (his name is Miles, and he's wonderful).
As I am not a desktop Linux user myself, I am not best placed to advise you how to configure your system for convenience in this regard, and will leave that to others. Best wishes to you.
In Ubuntu this is done the easiest by installing package r-base-core but you would probably want a higher version than the 3.4.4 that is shipped with Bionic
That is a pity, because if you did not it would have helped to install R - a quick and easy win
Just to double check that R is working correctly try running R in console - you should get a note about a Dark and Stormy Night; type q() to quit.
If R in console works as expected you will have confirmed that the issue is with RStudio. I would start with reinstalling (just in case) and if the easy way of the binary does not cut it than the long way of installing from source it must be...
Also, one thing you might want to check -- does your user account have permissions to access the folder located at ~/.rstudio-desktop? Fixing permissions there may help: