It’s not clear why the build failed. If you have installed the headers package and created the symbolic link:
sudo ln -s /usr/src/rbp2-headers-$(uname -r) /lib/modules/$(uname -r)/build
it should have built correctly.
However, I see that the T2U Plus has a VID:PID of 2357:0120 but the driver I built doesn’t have this combination, the closest being 2357:0122. There is a workaround to this issue but if OSMC will be including the 8812au driver in the next release, it’s probably worth waiting for it.