Market orders are blocked for deep ITM index options because they are usually illiquid contracts with low trading activity. The bid and ask spread could be very high and far from the contracts' last traded price or theoretical price. Market orders could get executed at a far away price, affecting your P&L. This happens because a market order is an instruction to buy the specified quantity of a stock at any available price.
You can use a limit order as a market order to overcome this issue.