A market order is a request to buy a stock at whatever price is currently listed.

Because OTC stock are soo illiquid and prone to doubling or halving their worth overnight, many brokers don't allow for market orders.

The alternative is to create a limit order. A limit order says only buy the stock if it is above/below a certain price. Your broker is forcing you to define the maximum you are willing to pay for the stock to limit your risk.