You get an error because it’s looking for function named as set_default_page_template while you seems to have function named set_page_template_default.
I’m not sure you either don’t have a function named set_default_page_template or you later renamed it. but that’s the error. WP cannot find your hook because it doesn’t exist.
you should either rename your set_default_page_template to set_default_page_template (if this is the function what you need) or you should create a new function and name it as set_default_page_template