A time can only be booked by players who have a price for it. To make times members-only, switch off the normal price for those times and add a price only for the allowed cohorts.
Steps
Go to Settings → Price Lists and open the price list the courts use.
Click ⋮ on the variant that covers those times, then Edit. Or click Add new price variant to create one just for those times.
Switch off For players not linked to a pricing cohort.
Under Discounted prices, click Add discount price.
In Cohorts, choose the cohorts that may book. Keep Minimum Duration on Any duration and enter their Hourly Rate.
In Opening days and times, set the days and times that should be members-only.
Click Save.
To make only some courts members-only, give those courts their own standard price list, because each court uses one list.
Check these traps
A discount row left on All players opens the time to everyone again.
Any other variant in the same list that has a normal price for the same times also opens them to everyone.
Hours you want open to everyone still need a variant with a normal price.
If a special period price list is active on the court, it replaces this list. Set up the members-only times there too.
Admin bookings on members-only times
You can't add a non-member to members-only time as a Court Booking, match or Recurring Booking, because that player has no price. Use a Bespoke Booking and set the price yourself.