[Limit Category to 1 Category in WordPress Edit Screen] #Taxonomy

functions.php

/*
* Limit Category to 1 Category in Edit Screen.
*/

function limit_cat_to_one() {
	if ( is_admin() ) {
		?>
		<script>
			jQuery( document ).ready( function ( $ ) {
				$( "#category-all input:checkbox" ).change( function () {
					var max = 1;
					var count = $( "#category-all input:checked" ).length;
					if ( count > max ) {
						$( this ).prop( "checked", "" );
						alert( "You can choose " + max + " Categor" + (
							max == 1 ? "y" : "ies"
						) );
					}
				} );
			} );
		</script>
		<?php
		
	}
}
add_action( 'admin_head', 'limit_cat_to_one' );