Sublime Text is fully functional in it’s unregistered evaluation mode with the exception that:
- It reminds you every few saves to purchase it if you’re going to use it for an extended purpose (i.e. it’s not free to use forever, despite what many web pages and users claim)
- You cannot turn off update checks at startup to gently remind you that as an unlicensed user, you should be running the most recent version.
The error that you’re seeing isn’t Sublime telling you that you can’t save because it’s unregistered, it’s MacOS telling you that you can’t save files to the root of the file system (/).
On Unix-like systems such as MacOS and Linux, non-administrative users aren’t allowed to save files anywhere but their own home directories (and a few other places, like /tmp) unless you specifically set things up to allow it, because indiscriminate creation or editing of files in the wrong place can cause problems.
As such, if you pick a different folder (somewhere under /Users/yourusername) to save in, it will work as you expect.