Cannot activate a manually-collected subscription

You can't activate a subscription where collection_mode is manual.

Error Message	`Unable to activate subscription where collection_mode is manual`
Error Code	`subscription_manual_collection_mode_activation_not_allowed`
Status Code	`400`

Common causes

You're trying to activate a manually-collected subscription, which is not possible.

Change the billing date of the subscription instead. Set the next_billed_at to 30 minutes after the time of your request, then wait 30 minutes for the subscription to become active.
Wait for the subscription to become active at the end of the trial period.