The trading platforms will prevent you from placing trades that would push your margin utilization above 100%.
On the trade ticket, a margin preview is provided to show how much Initial Margin is currently available.
If the Initial Margin required for the trade exceeds your available margin, the order will be rejected.
In that case, you can try reducing the trade size to fit within your available margin and submit the order again.
Note: The displayed Initial Margin does not include the impact of any existing open orders. These are only factored in during the actual order placement process.