Assuming the site is live, you have connected Stripe and you are based in the UK, selling in Pounds Sterling, then the option should be available. If you've checked these and are still experiencing an issue, you are encouraged to inform Squarespace directly by raising a ticket through Squarespace Customer Care because they don't use this forum to track bug reports.
If your "multiday cycle event" event is an event (and therefore a service product), you won't be able to use Clearpay for this. Clearpay can only be used as a payment option for physical products.
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