Some clubs limit how much each player can book, for example a number of bookings per week. When you reach the limit, you'll see Booking limit reached and can't book that time.
What you can do
Cancel one of your existing bookings, then book again.
Pick a different time or day.
Ask your club about its booking limits.
Member price limits
Some memberships include a limited number of member-price bookings. When you've used them, you can still book, but at the standard price. Checkout shows a message such as "Member price used for 2 of 2 sessions this week. The standard price applies to this booking."
Good to know
Limits are set by your club, and only the club can change them.