Did you purchase the download from Adobe? If so, the only one who can sort out this issue is Adobe.
Please go to its online Adobe Chat Activation support at the following link
Have you purchased the product within the last 14 days?
Please let us know the outcome.
Meantime, make sure you're entering the Premiere Elements serial number and not the Photo Elements serial number.
Premiere Elements serial numbers begin with 1143 while Photo Elements numbers begin with 1057.