Somehow your java version cannot find it. I had to make a special link to get this to work for java 8 oracle. It looks for native libraries in a certain place. From there you need to link to the rxtx library. Example on how to hunt down the location:
which java
/usr/bin/java
ls -la /usr/bin/java
lrwxrwxrwx 1 root root 22 Aug 31 19:48 /usr/bin/java -> /etc/alternatives/java
ls -la /etc/alternatives/java
lrwxrwxrwx 1 root root 39 Aug 31 19:51 /etc/alternatives/java -> /usr/lib/jvm/java-8-oracle/jre/bin/java
ls -la /usr/lib/jvm/java-8-oracle/jre/lib/amd64 | grep -i rxtx
lrwxrwxrwx 1 root root 29 Aug 31 20:07 librxtxSerial.so -> /usr/lib/jni/librxtxSerial.so
This last link I created by hand with ln -s. You may have to create a similar link for your java installation (obviously not under amd64, but whatever your CPU is, probably arm). You can find the location of the rxtx libs like this:
dpkg-query -L librxtx-java | grep .so