That’s not really a question, but as a not really an aswer, here are a few guidelines that you should keep in mind.
Code convention
Always try to prefix your functions with something unique (ex. your company name). This will be used like a namespace to avoid function name collision.
Database
Creating and updating a custom database table should be done in the plugin activation process, not during a custom function call.
register_activation_hook( __FILE__, 'pluginprefix_function_to_run' );
More information on the process here.
PHP
It looks like your code is defining a PHP function inside another. That’s not how you should do it. Functions should all be declared at the same level (or better, in a class).
WordPress
WP let you store almost everything you want in the wp_options table. If you need to add a custom config (like your admin field), you should try to store it in this table.
You can easily set and get options with
$value = get_option( 'my_custom_option_name' );
update_option( 'my_custom_option_name', 'new value here')
Link to the docs get_option and update_option
Next step for you might be to try to add this value somewhere in a page with a short code or in your theme.