I can't reply to buyer due to unsend pls help me
Do you mean you've been muted? If so, all you can do is wait. It's usually 1-2 weeks.
If it is truly a buyer (meaning they already purchased), you have their email so can contact them that way.
It sounds like you have had your messaging privileges blocked. Etsy does this when they detect that sellers have been sending unsolicited messages to other Etsy users, usually when they have favourited an item. You can only message a potential buyers if they contact you first. Otherwise it is considered SPAM which is forbidden (and illegal in some countries). Occasionally it happens by accident. There is nothing you can do about it other than wait for Etsy to decide you can start sending messages again (usually in a couple of weeks).
if you use the convo system to send out messages that are not in relation to an order, such as sending someone a coupon for hearting an item...that's a violation. They report your contact as spam...Etsy disables your ability to send messages. Normally, it is for a few weeks, but I've heard some folks say it can last over a month.
You won't be able to appeal this or work through this...just have to be patient. And don't do it again.