Codecademy have done a great job at reducing something that can be very difficult down to laymans terms but its difficult deciding just how far down the rabbit go they need to go.
While the problems with the REPL and trying to delete text are obviously a problem (and a hard one to solve), I'm not sure the issue with leaving out the quotes is. Obviously if the text was replaced with "I don't know anything called liz" instead of "liz is not defined" would be a minor improvement -- but at the same time anyone should be able to sit back, look at what they were asked to code ("Ryan".length) and notice the difference to what they typed (liz.length). The "Oops, try again" might be enough of a message there.
For some of these early lessons at least, could they not have 'curated' error tests?
ie. for this one, since missing the quote is an obvious error, why not match the string entered with a regex (or your parsing tool of choice) and look for the presence of exactly 2 quotes, with at least one character in between. If it's not there, print "That didn't work. Did you miss out a quote?"
In fact, if they log all the things people type in, they could analyse it for all the most common errors and then start attacking each of those.
While the problems with the REPL and trying to delete text are obviously a problem (and a hard one to solve), I'm not sure the issue with leaving out the quotes is. Obviously if the text was replaced with "I don't know anything called liz" instead of "liz is not defined" would be a minor improvement -- but at the same time anyone should be able to sit back, look at what they were asked to code ("Ryan".length) and notice the difference to what they typed (liz.length). The "Oops, try again" might be enough of a message there.