DATABASE_URL var not found when deploying to Fly.io

I'm deploying my first OpenSaaS Wasp app, but I'm running into the following error during the deployment process:
[ Db !] Error: P1012
[ Db !]
[ Db !] error: Environment variable not found: DATABASE_URL.
[ Db !] --> schema.prisma:4
[ Db !] |
[ Db !] 3 | provider = "postgresql"
[ Db !] 4 | url = env("DATABASE_URL")
[ Db !] |

The app still deploys, and I see my client, server, and database listings on Fly, but the app, of course, isn't functional.
I've tried changing my DATABASE_URL in env.server, adding my postgres url as a secret in Fly manually, etc, and have messed around with it quite a bit to no avail. Maybe (hopefully) I'm just making a very basic newbie mistake, but has anyone else faced this issue?
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