How to get category of current post

Following code can be used to get the category of the current post which you are dealing with

You can use the get_the_category( $post->ID ) to get the category. What you have pass here is the ID of the post. You can get the ID of the post using the $post global variable

global $post;
$postcat = get_the_category( $post->ID );

if ( ! empty( $postcat ) ) {
    echo esc_html( $postcat[0]->name );   
}

Use print_r($postcat) to see what is coming with category object