Yes, shih tzus can be left alone, but as they were originally bred as companions for humans they prefer to be around people most of the time.
Puppies shouldn’t be left alone for more than 2 hours, and even adult shih tzus, which can handle being alone for 4- 6 hours per day, may sometimes experience separation anxiety, and develop bathroom issues.
They thrive in households where people are around all day, or where they can develop a close bond with someone who’s always nearby. This is why they’re considered a great choice for senior citizens and those who can take their pets with them wherever they go.
In short, if you have to leave your dog alone for hours every day, for work or other reasons, a shih tzu will not be the ideal breed for you.