Your code does work. It’s just that you are declaring $xfield_args after you have made the call xprofile_insert_field( $xfield_args );
it should look like this
add_action('bp_init', 'field_xprofile_twitter');
function field_xprofile_twitter()
{
global $bp;
$xfield_args = array (
field_group_id => 1,
name => 'Twitter',
can_delete => false,
field_order => 1,
is_required => false,
type => 'textbox'
);
xprofile_insert_field( $xfield_args );
}