Give a look to vendor/magento/module-sales/Controller/Adminhtml/Order/Create.php file
You'll find some events there, such as adminhtml_sales_order_create_process_data_before, adminhtml_sales_order_create_process_item_before, adminhtml_sales_order_create_process_data, etc...
Depending by which exact moment you need to run your validation you may choose one or another
Another approach: I believe you could use almost all generic order events during checkout in frontend, but just placing your events.xml file in etc/adminhtml folder in your module. This way they will run only in admin requests