The place order button on your screen may be disabled because any of the following reasons:
Your KYC and onboarding is not completed, and hence you will not be able to place an order. In this case, you can simply finish the onboarding process and open your account.
You do not have enough margin or money in the trading account. In this case, you can simply add money to your Dhan trading account.
The scrip you are placing the order is restricted by Stock Exchanges or by the risk policy of Dhan.
If the place order button on screen is disabled, then you will see the appropriate message and reason for the same on the Dhan app or website.