Composer update error - site down :(

i followed this solution

so I ran: ANSIBLE_TAGS=composer vagrant provision from my trellis local directory.

but I have still have the same errors:

( ! ) Fatal error: Uncaught Error: Class 'Dotenv\Dotenv' not found in /srv/www/domain.co.uk/current/config/application.php on line  *17*

( ! ) Error: Class 'Dotenv\Dotenv' not found in /srv/www/domain.co.uk/current/config/application.php on line  *17*

am I missing something?

what else I can try to do?