The customer says the pay link does not work

A pay link does not expire and an older copy of the same link keeps working, so "the link is broken" almost always means the email client wrapped or cut a long link, or the customer is opening an invoice that has since been voided — the page says so in plain words either way. Sending the invoice again is the fix for both. If the customer can see the invoice but cannot pay on it, Stripe is not connected: Settings → Integrations → Stripe, after which the pay button appears; until then the page shows your pay-by-check instructions instead. A paid invoice's link opens to a paid receipt, never to a second charge.