There's a few reasons for this to happen: 

1. You have multiple instances of OpenVPN running, one in the front another in the background. Open taskmanager and check for any "openvpn" instances and close them all, then try again.

2. Or simply do this (Important Note: both commands must be executed as "Administrator") - if you dont have  below entries, simply proceed directly to step #3:

• Go to Start Menu -> All Programs -> OpenVPN -> Utilities -> Run "Delete ALL TAP virtual ethernet adapters"
• Go to Start Menu -> All Programs -> OpenVPN -> Utilities -> Run "Add a new TAP virtual ethernet adapter"

Another reason is you have older previous VPN softwares installed (e.g. HotspotShield, proXPN, UltraVPN) and they interfere with connecting and are using the adapter. Remove all other VPN software then do a reboot or reinstall OpenVPN freshly, so the adapter gets re-assigned to the latest installation.

3. Additionally you can simply disable + enable the tap32 adapter in order to reset the adapter to solve it: 

1. In Windows click Start, and then click Control Panel.

2. In Control Panel, under Pick a Category, click Network and Internet Connections.

3. Once in Network and Internet Connections right click the "TAP-Win32 Adapter V9" adapter and disable and enable it again (this disable/enable task usually solves the interface problem too).