Great, the first issue you will find, are there any shares to short. The next issue is if the security is marginable, if not then you are strictly on margin buying power only to borrow such non marginable shares. After you get through that you have to fund your account for Reg T requirements of $2.50 a share in Maintenance Margin. Did they also mention the fact that IB charges what is called "Supplemental Margin" of 102% of the principle amount?
This why shorting pennystocks is NOT a profitable trade, especially when you consider that you can only realize a total gain of 99% and the broker can pull your position at anytime. Further damage is done when a pump occurs and now you are on the hook for losses that can be infinite in theory.