Drivers not installing
I don't have any experience with the specific drivers, but here is how I would attack the problem.

I would get a USB drive with a Mac partition I can wipe - it should be 20 GB or so (10 might work as well) and I'd download a clean OS installer from the App Store and install a clean OS onto that test partition.

You can use startup disk preference pane to switch between OS and then don't restore any files to the test OS. Just make a test account and apply all upgrades recommended by Tascam (10.9.5 or whatever) and then install just the driver.

You'll then have a clean system to know if the issue is Tascam's to resolve and/or know you can try again with different OS levels if the issue is really a driver-OS compatibility issue.