A wallet is greyed out when it can't be used for that booking. The app doesn't show the reason, but it's usually one of the ones below.
The wallet's own rules
Clubs can limit a wallet to certain booking types, courts, days, times or dates. The whole booking must fit the rules. Check the wallet's description or ask the club.
Lead booker with Pay your part
When you book a court with Pay your part, the full court cost is reserved, so the wallet must cover all of it. It can't be combined with a card for this.
Example:
Your wallet has £20.
Your share is £8, but the full court cost reserved is £32.
£20 doesn't cover £32, so the wallet is greyed out, even though it covers your share.
The same applies when you book a league match as lead booker, or sign a team up for a league.
What you can do instead
Choose Pay full cost. The wallet then pays what it can and you pay the rest by card.
Pay by card for this booking.
Join someone else's booking, or a public or private match, and pay your share with the wallet.
Top up your wallet.
Other reasons
A wallet that has expired, hasn't started yet, has no balance left or belongs to another club isn't listed at all.
Memberships and club wallets can't be paid with a wallet, so the option doesn't appear.