Hi & welcome to LQ! Feel free to introduce yourself (&'distro', hw, goals&interests, etc)
here.
I think you need to run/invoke/execute the command/program:
mothur
first. See
https://wiki.gacrc.uga.edu/wiki/MOTHUR-Sapelo, which links to
http://www.mothur.org/wiki/Command_line_mode
Note that it is the Linux 'bash' (shell/command-interperter) that is complaining about your
make.shared(biom=example.biom)
which is a mothur command (not a bash shell command)!
bash gives similar syntax error IF you enter (at the % or # prompt) something like: man(anything)
Best Wishes... Have a look at some of the linux (&shell) tutorials links
here.