Array to String Conversion warning in shortcode

You are getting that error because in the function parameter you are passing $content as 1st parameter but as per add_shortcode callback function it accepts 2 argument

1. $atts – array
2. $content – string

It is basically trying to concat $atts param which is an array hence you are getting warning.

Just update your code by

function my_annoucement_shortcode($atts,$content)