I just ran into this after creating a brand new file and importing a previously created and fully functioning file. Went to save. "Could not save."
Unfortunately, because it was a brand new file, I could not find a recovery or temp file to open. Also, why even bother telling me what parameter is empty if I can't actually change that parameter?! That's like telling me I can't save the file because the name of the program isn't "Storyline 361."
Luckily I won't actually lose any work, this time, but these kinds of vague, unsolvable error messages are incredibly frustrating, especially since every single "cannot save" error message has been posted here, and every single official response is exactly the same and tantamount to saying "roll the dice and try again, maybe it'll work this time?"