instrument_in_use
HTTP 409, type conflict_error.
That card is still billing an active subscription.
Why it happened
Detaching it would leave the subscription with nothing to charge.
How to fix it
A subscription that is pending_first_payment, trialing, active, past_due or paused still bills against the card. Move it to another card with POST /subscriptions/{id} and payment_instrument, or cancel it now with POST /subscriptions/{id}/cancel and cancel_at_period_end: false. A cancellation at the period end, which is the default, leaves the subscription active until that boundary, so the detach keeps failing until then. Then detach the card.
Retrying: do not retry the detach while any subscription in one of those states bills against the card. Once the subscription is moved to another card, canceled now, or has reached the end of its period after a period-end cancellation, the same detach succeeds.