Subi does not have a "Charge now" button. There are two ways to get a charge to happen sooner, depending on why you want it.
Bring the next order forward
Open Contracts in your Subi admin and open the subscription.
On the Next order card, click Reschedule.
Pick a new date. The calendar greys out today and tomorrow, so the soonest date you can pick is the day after tomorrow. The next order date on the card is shown in UTC.
Click Save.
The Reschedule button is greyed out unless the subscription is active, and also while your account is over its plan limit. For a paused subscription, resume it first. How the dates after the new one follow is explained in charge your customers on specific days.
Retry a payment that failed
If the last payment failed, you do not need a new date. If your store has Smart Payment Recovery, the subscription’s page shows a Payment recovery card after a payment has failed. On the Subi and Subi Plus plans, click Retry now and confirm with Send retry now. Subi tries the card straight away and skips the next scheduled retry.
The card shows the latest payment recovery for this subscription. Retry now appears only while that recovery is still active. If you do not see it, the other ways to retry are in Failed subscription payments.