Why am I unable to buy stock options close to the expiry?

You will be unable to open new long positions in stock options during the last two days of expiry (Wednesday and Thursday) because the contracts may become due for physical settlement. If your option contracts are due for physical delivery, your margin requirements will skyrocket. Because stock options can be illiquid, it becomes difficult to exit the contract, increasing the brokerage firm's risk. Click here for more info.

Please keep in mind that you will be able to exit the open short options positions for the quantity you hold.