importing a translation language file

and in many positions of the server there are the 2 files: another-wordpress-classifieds-plugin-it_IT.po and .mo with my personal translations

What positions? Loco Translate won’t search the entire directory tree; it will look only in expected locations. You can add to these in the Advanced configuration tab.

yes of course, I used the 3 expected positions, the last one was
wp-content/languages/loco/plugins/another-wordpress-classifieds-plugin-it_IT.po
Anyway, let’imagine I want to edit the file in the following directory: /www/wp-content/languages/another-wordpress-classifieds-plugin/another-wordpress-classifieds-plugin-it_IT.po
what I should set in the advanced configuration? thanks for your help

You would set a relative path from the plugin being configured to the directory where the files reside. So in your example you would add the following as an extra domain path:

../../languages/another-wordpress-classifieds-plugin

I think the problem is not the position of the file. I open a translation file but without my custom translation. It’s invisible. But it’s runnung in the website, I just tried to publish a new ad and I can see my translations. Is there no paid assistance? Someone who looks inside and tells me what’s wrong – thank you

If Loco Translate can read your PO file, it will show the translations inside it. If it can save and compile a MO file, they will be on your disk for loading at runtime. Whatever problem you are having, you have not yet described any conditions that enable me to suggest what is wrong.

There is no paid assistance. I am here to help if your problem falls into the remit of the free support available, but you must describe the problem.