I had the same problem with my US-1641 but I did find a solution which may work for other units such as the US-200.
Basically Sierra does not seem to load drivers (kexts for Mac OS) that contain both a 32bit and 64bit binary and, at least for the US-1641 El Capitan kext, both binaries are present. See https://forums.developer.apple.com/thread/50380.
Luckily, code signing seems to be done on each binary when it is compiled and removing the 32bit version allows the kext to be loaded.
The following steps worked for me but obviously your mileage may vary.
Usual disclaimer: I accept no responsibility for what may happen to you, your computer, peripherals or anything else that may occur from following these steps. Use strictly at your own risk!
Messing up with kernel extensions could prevent your machine from booting or cause crashes so make sure you have all work saved, a backup and are ready to restore/fix your machine from the recovery partition. If your are not comfortable performing the steps outlined, reverting to El Capitan is a safer option.
In the instructions below you will need to replace all instances of 'TASCAM_US1641' with whatever applies to your device.
Open a terminal.
Ensure that you can proceed further by checking that you see information about x86_64 and i386 from the following command:
file /Library/Extensions/TASCAM_US1641.kext/Contents/MacOS/TASCAM_US1641
If so, remove the i386 binary with the following:
sudo lipo -thin x86_64 -output ~/Desktop/TASCAM_US1641 /Library/Extensions/TASCAM_US1641.kext/Contents/MacOS/TASCAM_US1641
This will have output a file named TASCAM_US1641 to the desktop which should only have the x86_64 binary.
Remove the orignal:
sudo rm /Library/Extensions/TASCAM_US1641.kext/Contents/MacOS/TASCAM_US1641
Copy the new binary (thanks to gizabj for adding the missing 'sudo' here):
sudo cp ~/Desktop/TASCAM_US1641 /Library/Extensions/TASCAM_US1641.kext/Contents/MacOS/
Try loading the new kext:
sudo kextload /Library/Extensions/TASCAM_US1641.kext
There should be no errors at this point. You can use kextstat as an additional check here.
Now rebuild the kext cache:
sudo rm -rf /System/Library/Caches/com.apple.kext.caches
sudo kextcache -prelinked-kernel /System/Library/Caches/com.apple.kext.caches/Startup/kernelcache -K /System/Library/Kernels/kernel /System/Library/Extensions
Reboot your Mac, delete that file from your desktop and assuming everything went fine, get back to using your Tascam
