Hi Stefano,
What operating system are you using?

What version of the date utility are you using?

The command head n- 1 in your pipeline is always going to give you an error unless you have two files in your current working directory named n- and 1. I assume that you intended to write head -n 1 instead of head n- 1.

The meaning of the date -d option varies depending on the operating system you're using and the version of date you're using.

I would guess that the code is attempting to print the name of the last file in the directory named by the contents of the BACKUPS_FOLDER variable whose name contains a date of the form YYYY-MM-DD where YYYY is last year. Whether or not it succeeds will depend on the answers to the questions above and the names of the files in that directory.