I wrote a Mathematica notebook and a package using Mathematica 11. Everything works fine there, so I tested it also with Mathematica 10.4 in my computer. As I did with Mathematica 11, I click Evaluate Initialization Cells (to make everything in the notebook ready for the user) and everything works. So, I sent my files to my teacher, who uses Mathematica 10.4, and he said me that, after clicking Evaluate Initialization Cells, everything stops working because (he guessed) the package cannot be found. I load it at the beginning, putting this code in a initialization cell:
SetDirectory[NotebookDirectory[]];
<< IlNostroPack`
Why is the problem happening to him? I can send you a google drive link to my project, so you will be able to test it on your computers.