Why can't I place/edit a limit order?

In order to place a limit order, you must have enough balance to satisfy the whole order. For example, if you want to buy 1 BTC with limit price at $4000 USD, you must have $4000 ready as free balance. Similarly, if you edit a limit order but the change will result in you not having enough balance to satisfy the whole (updated) order, system will not allow the edit to take place.

Free balance here indicates the remaining of your balance in a fiat currency account after counting used margin, order margin (free balance of other limit orders, if any), pending withdrawals, negative P&L, and live loan offers.