The token may have expired because the email was first sent a while back.
Steps to resolve this error:
1. Log in to your Pencilpay Account.
2. Go to Customers -> Search for your Customer.
3. Click on More Actions (...) and select "Send Email".
A pop-up will appear where you can type in the email address and hit Send.
4. Re-send the email to your customer and ask them to click on the new link.