Originally Posted by
ms_go
Now we have a second problem in that we have to complete an entry form for Spain and get a QR code. However, the form for Spain cannot be finalized and the QR code issued until all fields are complete, and that includes a seat number on the Lufthansa flight. And since we are in economy on that flight, we can't get seat assignments until check-in. So it looks like we'll have to check in at the airport in anyway.
I went through the same process recently. I was trying to do online check-in and needed the QR code to complete it (this was on Air Europa not United). I ended up making up seat numbers and uploading the QR code to check in and receive my seat assignment. I received a confirmation email from the Spanish registration site, and it included a link to edit my information. I changed my seat number to the actual one although I don’t think anyone would have noticed or cared if I didn’t.