Helper - Call to undefined function

It's not very clear in the documentation, but when you create a new helper, the filename must have the '_helper' suffix: https://codeigniter4.github.io/userguide...g-a-helper

So rename your file to build_page_helper.php and it should work. To load the file, you don't need the suffix, so keep this line as is: 

PHP Code:

helper('build_page');