If you've successfully sent the skin, but the offer has been cancelled later, there are several reasons for that:

1. An increased load on Steam servers

The skin status in the app should be updated within 1-2 hours, so you can try to withdraw it again.

2. This skin has been accepted by another user

It sometimes may happen that the same skin is being requested by several users. If your trade offer has been cancelled, it means the offer has already been accepted by another user.

Don't you worry; the status of your skin should be updated within a couple of minutes. Try sending it again.

3. You've mistakenly cancelled the order yourself

There's nothing to worry about as well. Wait until the status of the skin is updated in the app and try sending it again.

While the second and third issues are fixed relatively easily by repeating the withdrawal, the first one depends on Steam servers. Usually, it takes them a couple of hours to fix this, but sometimes it can take several days.