There are a few reasons why you may not be able to change templates in Squarespace.
First, some templates are specific to a certain type of content, like a blog post or gallery. Changing the template for that type of content may not work as expected.
Second, some templates are based on specific content or design elements, and changing them may cause instability or errors in your pages.
Finally, Squarespace may not allow you to change a template because it is associated with a protected piece of content, like a blog post or page layout. In some cases, you may need to contact Squarespace support to change the template.