Where is your
events.xml situated? In
etc/adminhtml? If it's only in
etc/frontend you need to move you
etc, or add an
events.xml-file to your
etc/adminhtml-folder with a similar event.
Another thing could be that the checkout is indeed something that is frontend-only. If you're using the admin, you're not using a checkout, but you're creating a quote / order. In that case you might want to take a look at
Magento\Sales\Controller\Adminhtml\Order\Create\Save(). This is the controller that handles the saving of an order from the admin and might be a good starting point.
If I'm running through the code I finally come to a method called
Magento\Sales\Model\AdminOrder\Create::createOrder(). This order dispatches an event called
checkout_submit_all_after, but because it's a public method as well you could also use a plugin / interceptor for this, depending on your needs. Make sure to put your
events.xml in the
etc/adminhtml-folder, otherwise they might collide with your previously checkout-related events. It could be even possible that if you hook into this method, the checkout-events can already be obsolete.
I hope this helps you a bit. I haven't tried it out, but I think it should do the trick.