I emailed them and got this response:
Dear Mark,
Thanks for taking the time to email us here at TrialPay.
Sorry you've been having trouble accessing our offers. Our inventory is
constantly changing, and it's possible that the offer you chose is no longer
available at this time. Additionally, please be aware that most of our offers
are only available for completion once per household.
If you're still
unable to access a particular offer after waiting 24 hours, please select an
alternate offer to complete.
If you have any other questions or
concerns, please let us know.
Regards,
Julia
TrialPay Customer
Support