Typed a comment, pressed enter, and had this pop up. Comment still went through and seemingly nothing happened, but first time seeing this message.
I find the iNat pages ‘decay’, so if I’ve started typing a comment (or adding an ID) and then go off and do some research, or have friends pop in, or see an interesting wasp on the euphorbia, or for whatever reason I’m away from the comment for a while, when I come back and complete it, I get the ‘failed to save’ popup (and it doesn’t save!!). I have to then refresh the page and start again. For this reason, when I type long comments, especially with keys or diagnostic info or whatever, I always copy/paste it into a text document before saving, so that, if the comment doesn’t save, I have not lost all my work.
Having said all that - I don’t believe I’ve seen the ‘parent can’t be blank’ parameter on the popup ever.
Looks like the observation doesn’t exist any longer: https://www.inaturalist.org/observations/35192155 So it’s likely it was deleted by the time you hit Save.