I could be wrong on this, but I recall reading not long ago a change was made where you had to purchase Samsung Care+ at the same time you purchase the device now. Otherwise you will be unable to join
Try taking a look at this link here: https://www.samsung.com/us/support/samsung-care-plus/faqs/
If you're still having trouble. We have a specialized team that can assist with this, Please call, 1-800-726-7864.