What are you using to make your style.css file?
The only thing I can think of that I"ve seen happen before is that folks make style.css using notepad/notebook
Which is not a problem by itself. But, they usually have extensions hidden on their machines, and do not check 'all files' when saving.
So the file looks as though it is style.css when it's actually style.css.txt
I just want to make sure something like that isn't happening for you
Also, if you are getting the 'missing stylesheet' error, it doesn't matter what code you have in style.css right now, as the file isn't even being recognized as existing....
A child theme will accept a preexisting stylesheet... any file named style.css works (as long as the header is in there, which you have). You should be able to freely edit it through the WP editor, through FTP, a file manager, etc.
Also, the folder you have created your child theme in... what's it named? Does it have any spaces in the name or anything odd?