I am somewhat new to LAMMPS as well, but I think I may be able to help you. First, you should check to see if you are in the directory you have the file installed in while you are running lammps, assuming it’s installed in the system path.
Otherwise, I think you might be having a problem because you are using Microsoft Word to input the file into. You should really be using a text editor like Notepad, which is included on all Microsoft systems, or Notepad++, which is open source software you can get on the internet that has better text formatting features if you are looking for making code scripts like the input file for LAMMPS.
If you input or paste the text right into Notepad and save it as insert_filename_here.in, you should be fine. You shouldn’t have to worry about using the special file formats Word gives you, if that is what you are using.
I hope this helps.