Links to orders in emails are signed and expire. There are two different messages, with different fixes.
"This link has expired"
The signed link to a receipt or invoice has run out. Receipt links last 30 days (security.order_link_days) and invoice links 30 days (security.invoice_link_days).
No action is needed from you. The page the customer lands on has a button that emails a fresh link to the address on the order. A customer with an account can also sign in and open the order from Account → Orders, where links never expire.
"This payment link has expired"
This one is on the pay page, and it is the order's own payment deadline: 7 days from when the pay link was issued (orders.pay_link_days).
- Open the order in KahunaCart → Orders.
- Click Reissue, or Reissue & email to send the customer the new link.