I do not do margin trading but I dont think so.
Can do margin trading or not depends on your account type. If you open a margin trading account, you should be able to do it.
I believe IB calculate your purchasing power based on your holding. The purchasing power is the value you can "buy" after margin effect.
Margin is a concept that you buy the asset even you do not have enough money.
Eg: you have 10k USD but buy 20k USD bond. Then you are doing margin trading with borrowing 10k USD.
Again this is what i guess as i do not do margin trading. Also, I do not recommend to do margin trading before knowing the potential risk.