Of course you can! When beginning a new business starting with a template based solution like Squarespace can get you off the ground with limited effort. However, there are some disadvantages. You are limited to editing the aspects of the website your chosen template allows you to edit. If the template does not include every type of page you need, you will have to figure out how to make that page yourself.
The most meaningful downside to this option is the time commitment. You still have to learn to use the editor you have chosen. This often means 30+ hours filled with confusion and frustration that you could be spending on other areas of your business or with clients. 30 client hours is a lot of money!