I am unable to print a scan form for labels that I printed out today.
Probably because it's Sunday.
It's Sunday, not available today. You may also have trouble printing it tomorrow, because your labels are dated today. You can try it, but don't be surprised if it doesn't work.
"It is only possible to print a SCAN form for shipping labels that have both a purchase date and ship date of today. It is not possible to print SCAN forms for labels purchased on different days. "