AMO’s can be placed only after 4pm .
This is because there's something called an "After market session" held between 3:30 and 4 PM. After market orders can only be placed after 4 PM. Check out this blog from Zerodha to understand market timings.
after market orders which we place are placed when markets are not live . i.e. after 4 pm and before 9:14 am of the next day. and in zerodha as far as i know, the minimum amount you need to place in after market order is 25000, you can do check it out my mailing zerodha at their email id: [email protected]. if your order is below 25k then it may not get executed. do check out minimum limit for placing after market order before placing such order.