Add code to your child theme’s functions.php file or via a plugin that allows custom functions to be added, such as the Code snippets plugin. Avoid adding custom code directly to your parent theme’s functions.php file as this will be wiped entirely when you update the theme.
https://gist.github.com/woogists/9a8f240c37d488783028a43a2416a647
The WC Minimum Order Amount plugin is based on this snippet, if you’d prefer to use a plugin.
Alternatively, you may want to try Minimum Purchase for WooCommerce (not an endorsement or recommendation).