Tap pay now on the unpaid payment in Profile → Purchases, or use the Pay button in the email from the club. Both take you to a checkout where you pay as normal.
Pay in the app
Open the Profile tab.
Tap Purchases. It shows an Unpaid badge.
Tap the payment with (pay now).
Tap pay now and pay at checkout.
You can also open the booking and tap pay now next to "This is unpaid".
Pay from the email
Tap Pay in the email. It opens a checkout on the player website, where you can pay by card, Apple Pay or Google Pay.
Why is it unpaid?
Usually the club added you to a booking, a regular weekly booking or a league team, and you pay your own share. A membership renewal that failed also shows as unpaid.
Good to know
If a checkout link has expired, find the payment under Purchases instead.
On a court booking where players pay their part, any share still unpaid about 4 hours after the session ends is charged to the lead booker.
If you think you shouldn't have to pay, ask the club.