In order to use WhatsApp Shared Number you have to first send a message with a specific keyword as mentioned in your "Settings -> Channels -> WhatsApp Shared Number" this will open a session for the next 24 hours i.e you can able to send messages out using the shared number to the specific number for next 24 hours, if you would like to reset the session then simply send any message to the shared number.
This is because you don't have enough balance to send messages out or your plan limits are exhausted , if you are sending a large number of messages try to decrease the count of numbers so you may able to send messages as per the plan limit or available balance.