This is a fluke that happens sometimes. Overall, it's simplest/easiest to just start over. Under the TOOLS tab on the left, select that option to clear and start over. This will completely clear *EVERYTHING* from your 2018 return, and you'll be starting it over from scratch, which will allow you to attempt the import again.
After you import *AND* get past the personal info section, you can check that the rental data (and anything else of concern) was imported before you continue further.
Please note that if you have already registered or paid your TurboTax fees, then the ability to clear and start over *NOT* an available option. If this is your case, post back and I'll give you the only other way to deal with this manually. It's a real pain too with the online version...but doable.
