This is because the same function exists in the parent.
archi_theme_setup()
Since you are under the same namespace there will be a name collision — PHP cannot hold two functions with the same name.
When you set the child theme as the active one child theme functions.php will load first and then the other one frorm the parent.
You cannot just copy paste the code.
Check the theme developer to provide you the child theme functions.php template.
Or use the empty template with the comments part in front.