I have just done a fresh installation of OSMC and Tvheadend and I think the most likely thing is you need two firmware files to make the adapter work. You can get them here. They need to be in /lib/firmware/ on the Pi. If you know how to use ssh then the easiest way to get them is:
cd /lib/firmware
sudo wget https://github.com/OpenELEC/dvb-firmware/blob/master/firmware/dvb-tuner-si2158-a20-01.fw?raw=true -O dvb-tuner-si2158-a20-01.fw
sudo wget https://github.com/OpenELEC/dvb-firmware/blob/master/firmware/dvb-demod-si2168-02.fw?raw=true -O dvb-demod-si2168-02.fw
HTH