It looks like your DB hasn't been initialised properly. The error you've included is failing to count the items in the User list which (if you don't have sessions configured) is likely the first query to run – a count of items in each list is shown on the Admin UI the landing page so that's the first thing it does.
So something about how your migrations are being generated or applied in production isn't setup right. Most of the relevant docs on how this works are in the CLI guide, specifically, see the section about database migrations and the db.useMigrations flag.
Having db.useMigrations turned off can be handy if you're just playing around in dev. Keystone will automatically sync your DB structure to what's defined in your list configs whenever it starts, and does so without creating any physical migration files. If you're prototyping some change or just mucking around, this may be what you want but – if you're deploying somewhere – better to turn db.useMigrations on. Then, if Keystone detects changes to the DB when it runs, it'll prompt you to create a migration file, which can be tweaked to protect existing data if needed, tracked under version control (eg. git) and deployed.
Getting these migrations to run in an environment like Heroku is a little slightly weird as (assuming it's enabled for your app) Heroku can auto-scale. Migrations on the other hand need to be run exactly once. You also can't just lock the DB and run migrations when the first instance of the app starts – this delays the start up of the HTTP server so, if the migrations run for too long, Heroku may think the deployment has failed.
The way we suggest getting around this is to run migrations in the build staging. Fans of the 12-factor app methodology will notice this violated the separation of build and release stages but, for a simple Heroku deploy, it works fine. For larger/more serious apps, creating and applying migrations usually an area that needs significant thought and attention. The specific infrastructure and rollout processes required will be project dependant.
I'd also encourage you to check out the Keystone 6 Heroku example codebase if you haven't already. It's a little out of date but it shows the migrations and package.json scripts in action.
PrismaClientKnownRequestError: The table `public.User` does not exist in the current database.
An error occurred handling a request for the Admin UI: Error: Prisma error: The table `main.User` does not exist in the current database.
PrismaClientKnownRequestError
[railway-deployment]: The table `public.User` does not exist in the current database.
I tried to deploy a keystone app to Heroku and I did it but while I tried to open the app I got the following error:
An error occurred handling a request for the Admin UI: Error: Prisma error: The table `main.User` does not exist in the current database.
Here's a screenshot containing more details about the error: