If your customer is trying to setup a paypal subscription, but the button for subscription is missing, then the invoice is likely overdue. WHMCS removes the option to setup a subscription if the invoice is overdue to prevent customers from setting up automatic payments that are going to always be late.
You can either change the due date on the invoice to today, or ask the customer to pay this invoice as normal, then on their next invoice, pay it before their due date with the subscription button.
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