To receive and pay support payments on-line, the non-custodial parent must be a Premium subscriber:
When a support payer receives a support payment request from a free / starter account, the PayPal payment instructions are blank. The payer can still approve or deny payment requests, but cannot make payments on-line via PayPal:
When a support payer receives a support payment request from a Premium account, the PayPal payment instructions are shown:
To allow your support payments to be paid on-line and marked as paid automatically, simply upgrade to Premium: