Google.Close is a column of myData, it doesn't exist on its own, you have to reference it as myData$Google.Close.
Alternatively, use the "x" and "y" vectors you have defined.
x <- myData$Google.Close
y <- myData$Amazon.Close
plot(x, y, type="l", main = "GoogleAmazonStock", xlab="Google.Close", ylab="Amazon.Close")
Or use the "formula" syntax and the "data" argument
plot(Google.Close ~ Amazon.Close, data = myData, type="l", main = "GoogleAmazonStock", xlab="Google.Close", ylab="Amazon.Close")