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merchants.bank_account_in_usemerchantsRule R8That bank account cannot be removed yet
Removing the default account of an active merchant is refused while a payout of that merchant is pending, because the payout is already addressed to it.
Why the API refuses
WHEN RemoveBankAccount is executed on the default account of an active merchant, THE SYSTEM SHALL reject it while a payout of that merchant is pending.
What to do
Wait for the pending payout to settle, or add another account and make it the default first, then remove this one.