Be sure you’ve committed your updated composer files to the repo that is cloned during deploy, like Downgrade WP when deploy - #6 by swalkinshaw. Then the deploy will run composer install automatically and install your plugins.
Re: the admin_user (named admin by default):
The passwords generated here are
example_passwordandanother_password, respectively. (Trellis Security docs)
So, if/when you need to run sudo commands, connect via SSH as admin and use the password example_password (or whatever you may have changed it to) instead of copying/pasting this value.