Do you see following warning in your build log?

[WARNING]: !Failed to set up process.env.secrets

Secrets in AWS amplify are very confusing with Gen1 vs Gen1, Frontend vs Backend, build-time vs run-time.

I found that this works:

• https://docs.aws.amazon.com/amplify/latest/userguide/environment-secrets.html#set-environment-secrets

plus you also need and IAM Service role with permissions to read from SSM parameter store, namely ssm:GetParametersByPath.

See this immensely helpful comment on github issue:

• https://github.com/aws-amplify/amplify-hosting/issues/3348#issuecomment-1476044689

If you did everything right, you shouldn't see that warning in the build log.