If you would like to manually send a payment request click on action & hit email on invoice page.
To start using this feature head to Setting >> PayPal Integration & enter your PayPal email details.
1- Check the box in front of 'Enable PayPal Integration'
2- Enter your PayPal email address.
3- Select currency
4- If you would like customers to receive payment link automatically update this trigger "Send payment request automatically when a ticket is marked as repaired"
5 - Hit "Save changes" and you're done.