The error: This trade would exceed your (initial) margin available

The platforms will not allow you to place trades that would raise your margin utilization above 100%. In the trade ticket, there is a margin preview available, indicating the amount of initial margin available. If the initial margin impact exceeds the available initial margin, the trade will be rejected.

Please, be aware, that the initial margin is shown without taking into account existing open orders. Existing open orders are only calculated during new order placement.