Runtime problem(s)

I “created” a theme basically following the instructions at “https://www.wpdesigner.com/2007/02/19/so-you-want-to-create-wordpress-themes-huh/”. I have two problems, though. First, my “home” (index.php) page does not show in the sidebar with the “about” page. Second, when I click on the page title, I get the following error:

Warning: Cannot modify header information – headers already sent by (output started at C:\xampp\htdocs\wp\wp-content\themes\skeleton\functions.php:1) in C:\xampp\htdocs\wp\wp-includes\pluggable.php on line 892

Any guidance will be appreciated. Thanks!!

PS: I’m running the theme with XAMPP locally and at https://cangrejero.com.