UPDATE 09 March 2016. Audacity has updated our source code to the latest PortAudio sources (r1966).
We would like anyone on Windows 10 who had to use Audacity 2.0.3 or earlier to avoid the Internal PortAudio error to see if the latest Audacity "nightly build" now recognizes all your devices.
The latest Nightly Build for Windows can be found at the top of this page: http://gaclrecords.org.uk/win-nightly. Note that Nightly Builds are for testing. They are not finished releases and should not be used for production work.
Please start a new topic to give us your feedback.
Audacity 2.1.2 does not have full support for Windows 10 yet because of driver requirements. It is essential that all audio devices including those externally connected have proper Windows 10 drivers matched for the device. That apart, Audacity works fine on Windows 10.
If you have not already done so you should go to the web site of the computer manufacturer or motherboard manufacturer and see if you can find the latest audio drivers for Windows 10 meant for your specific computer model. If new drivers are there, these might be better than the audio drivers that Windows 10 found itself.
The situation is fluid, so also keep using Windows Update. Use Windows Device Manager from time to time and try the right-click option over the audio device to update the drivers. It is possible that you will receive a bug fix for older drivers this way that might solve the problem.
If you are using external audio devices or interfaces that have manufacturer-supplied drivers or firmware, also go to that manufacturer's site and look for drivers and firmware for Windows 10. In some cases it is possible that disconnecting external devices one by one and restarting Audacity might enable Audacity to recognise the built-in audio devices.
Please see http://wiki.audacityteam.org/wiki/Windows_10_OS for fully detailed help.