sqlplus: error while loading shared libraries: libsqlplus.so: cannot open shared object file: No such file or directory

As seen in the comments your Library is located at /usr/lib/oracle/12.1/client/lib/, but your path just includes /usr/lib/oracle/12.1/client/bin... you see why it can't find the library?

You can expand your LD_LIBRARY_PATH with the /usr/lib/oracle/12.1/client/lib/ directory (in your .profile or .environment or .bash_profile or (if you want it for all users) in the /etc/bash.bashrc file) - i don't know if sqlplus takes care of the LD_LIBRARY_PATH - just have a try.

To see to what LD_LIBRARY_PATH is set you have to type:

echo $LD_LIBRARY_PATH

(note the $ if you want to read the value!)

Hope this helps!