I'm brand new to RStudio and unable to run some initial code. Seems like maybe something did not install correctly, but no solutions I've found have helped. When I try to plot a dataset, the following code results in the following error:
If you paste more of your script it could help someone figure out what's going on. Also it's hard for us to tell what mydata refers to. If you saved the diamonds dataset to mydata, then this should work if ggplot2 is loaded. But if mydata has different variables in it (instead of carat, price, clarity) you would need to update those mappings.
Also just an observation that you used aes=(stuff) which should be mapping = aes(stuff) (and really in ggplot you can get away with omitting both data= and mapping=).