Suffering from this in Windows 10?

OpenVPN: An error occurred installing the TAP device driver.  

or perhaps a variant of the above?

OpenVPN: Write to TUN/TAP : Unknown error (code=122)

Then install a new version of the OpenVPN client application.  You may still get the above so open up device manager.  Look under the Network Devices section.  An Unknown device should be listed.  Right-click it and let windows find an appropriate driver for it.

The end result should look like this:

Then again the above might not solve all the issues.  In case you get the error:

OpenVPN: Write to TUN/TAP : Unknown error (code=122)

It's likely due to this:

Sun Dec 29 19:10:49 2019 WARNING: 'link-mtu' is used inconsistently, local='link-mtu 1571', remote='link-mtu 1572'
Sun Dec 29 19:10:49 2019 WARNING: 'comp-lzo' is present in remote config but missing in local config, remote='comp-lzo'

Then uncomment this:

# Enable compression on the VPN link.
# Don't enable this unless it is also
# enabled in the server config file.
comp-lzo

Hope this helps!

Cheers,
JB