Start by using the debugger (https://www.myonlinetraininghub.com/debugging-vba-code
] will help if you have no idea how to do that) and look ate exactly what the string concatenation generates:
percorso & "/" & nomefile
It contains two variables, so it's important to find out what the resulting string is, exactly - assumptions and guesses don't count here!
When you have that string, first check it is a valid path, and then look at the path to the file it represents using windows explorer and check that the file and path all exist, and that the user running the code has the correct permissions to access the full path. Then check that the file is actually an excel file ...
Sorry, but we can't do any of that for you!