How to make the first letter of a post title uppercase, in a plugin?

As maverick said in the comments, you need to accept the arguments which come along with the filter.

Try this

function shikharfirstletter($title) {
    $title = ucfirst($title);
    return $title;
}
add_filter( 'wp_title', 'shikharfirstletter', 10, 1 );

https://developer.wordpress.org/reference/hooks/wp_title/