"Trading is disabled" means that the broker is not ready to execute order.
With LMax, please verify that you have access to the web API. Just contact LMax and ask them to activate the .NET API for this account, they exactly know what they have to do with this request.
With MT5, if your order has not been executed and the last status is "Trading is disabled", it could mean that you have connected your Slave account with the investor password instead of the trader password.
Please double check if you have used the correct password (trader password) to connect your Slave account.