The variable you added to that service has not been deployed yet. It is sitting in your environment's staged changes (the pending changes bar), so the running container never received it. It is also saved under the name REDIS_PRIVATE_URL, not REDIS_URL, so code that reads REDIS_URL will not find it even after it deploys.

In the service's Variables tab, add REDIS_URL with a reference to the Redis service, for example ${{YourRedisServiceName.REDIS_URL}} using the exact name of the Redis service on your canvas. The autocomplete dropdown in the value field fills this in for you. Then apply the staged changes with Deploy.

Before you deploy, review the pending changeset: it also includes deleting an older Redis service and its volume that you removed earlier, so dismiss that part if it was not intended.