Are you getting the error "Oracle VM VirtualBox - Failed to open/create the internal network" with VERR_INTNET_FLT_IF_NOT_FOUND for the VirtualBox Host-Only Ethernet Adapter while logging into a virtual machine created in Oracle VirtualBox with Windows 10 operating system ? If so, we already had one solution article in http://blog.dbauniversity.com/2016/02/oracle-virtualbox-failed-to-open-create-the-internal-network.html
If the solution in http://blog.dbauniversity.com/2016/02/oracle-virtualbox-failed-to-open-create-the-internal-network.html does not solve the problem (or) If you don't even find Virtual Box hosts-only adapter in Control Panel -> Device Manager -> "Network Adapters", then try the below alternate solution for Windows 10 Operating system.
1) Uninstall the existing VirtualBox software from the CONTROL PANEL.
2) Install new VirtualBox 5.2.44 in Windows by downloading https://download.virtualbox.org/virtualbox/5.2.44/VirtualBox-5.2.44-139111-Win.exe
3) Delete the default Host-only network created by this VirtualBox 5.2.44
4) Create a new Host-only network (Don't change the default IP addresses).
5) Now change the virtual machine's Settings => Network to use this new "VirtualBox Host-Only Ethernet Adapter".
6) Now retry starting the virtual machine and this time it must be successful.