The error "All TAP-Windows adapters on this system are currently in use" suggests that there is a problem with the virtual Tun/Tap network adapter used by OpenVPN on Windows. To solve the issue, you need to re-enable the adapter or reinstall the driver.
Solution 1: re-enable the adapter
Open Control Panel > Network and Internet > Network and Sharing Center > Change adapter settings.
Check description of network adapters to find the Tap-Windows Adapter.
Right click on it > Disable.
Right click again > Enable.
Solution 2: reinstall the Tap driver
Check if it is already installed by opening C:\Program Files\TAP-Windows
Right click on Uninstall and select Run as administrator
Download the Tap driver from OpenVPN repository and install it (right click on it > Run as administrator)