Why could I not cancel or modify my Market-On-Close (MOC) or Limit-On-Close (LOC) order?

You can select Market-on-Close (MOC) or Limit-on-Close (LOC) as the order type for a One Cancels All (OCA) group or for a Profit taking leg attached to a Parent order. Your Market-on-Close or Limit-on-Close order is given to the NYSE at 15:45 EST if the NYSE is the Primary exchange or to the Nasdaq (Island) at 15:50 EST if the Nasdaq is the Primary exchange. After that time you will be unable to Modify or Cancel that MOC or LOC order…

Many exchanges charge fees to their members for orders that are cancelled or modified, and IBKR charges certain related fees to its customers to help cover these exchange fees. Please note however, that the Modify/Cancel fees listed are not intended to be a direct "pass-through" of exchange fees. The Modify/Cancel fees that IBKR charges to its customers may be higher or lower than the fees that an exchange may impose on IBKR. IBKR may charge Modify/Cancel fees to its customers for orders sent to exchanges that do not impose such fees, or vice versa…

Market-on-Close (MOC) orders are supported on the LSEETF exchange, but only if you direct your order to LSEETF instead of SMART. To direct your order to LSEETF in TWS, click on "SMART" and change it to "LSEETF".

Please note, this order type may only be supported in your live trading account and not in your paper trading account…

The BATS exchange cut off time is 15:55 EST to submit a Market-on-Close (MOC) or a Limit-on-Close (LOC) order for consideration to be included in the closing cross. MOC or LOC orders submitted after 15:55 EST can be rejected, and you would receive a message stating "On-close orders cannot be placed during or near the closing cross"…