Same here, this is a bug in TurboTax 2019, and as far as I can tell, would be something you would get a letter (or worse) from the IRS about if you did get a 1099-S and the field was left blank. The 1099-S says "If you are required to file a return, a negligence penalty or other sanction may be imposed on you if this item is required to be reported and the IRS determines that it has not been reported". Sounds important to me.
While I was able to enter the value for line 1 directly on the form via the Forms tab, I'm unsure if such changes are persistent; if it gets reset later for some reason I'll update this comment.
