Conditional Statement custom post type category

is_category() does not work on custom taxonomy archive pages. The correct conditional tag here is is_tax() which takes the name of the taxonomy as first parameter and a string|int|array of term name/s, slug/s or ID/s as second parameter.

So your whole conditional statement can look something like :

if ( is_tax( 'my_taxonomy', 'slug-name-of-the-term') ) {
    echo 'My text';
}

EDIT

In addition, to test whether a post belongs to a specific term, you should use has_term() to test for the specific term

global $post; 
if (    ( $post->post_type == 'myposttype' ) 
     && has_term( 'slug-name-of-the_term', 'my_taxonomy' )
) {
    echo 'My text';
}