The trading platforms will not allow you to take trades that will bring your margin utilization above 100%.
In the Trade Ticket you will have a margin preview available, which gives you an indication of how much Initial margin available do you have.
If Initial margin impact is higher than Initial margin available, your trade will be rejected.
Please, be aware, that initial margin is shown without taking in the account existing open orders. Existing open orders are only calculated in during a new order placement.
As you can see in the example below, Initial margin impact is USD 8.501,14 and it is significantly higher than Initial margin available of USD 2.633,40. As a result, the trade is rejected.
Please, reduce a size of your position and try again.