For Capital One (ins_128026), /transactions/refresh is not supported on Items that include only non-depository accounts (e.g., credit cards), and calling it on such an Item will return a PRODUCTS_NOT_SUPPORTED error. Capital One depository accounts (checking, savings) do support /transactions/refresh.
For Items where refresh is not available, use /transactions/sync to fetch transactions for the affected accounts; new and updated transactions will surface as part of Plaid's regular update cadence.