I had similar issue. But setting the environment variables (KEYCLOAK_ADMIN and KEYCLOAK_ADMIN_PASSWORD) did help.
But in my case I was having the distribution under user directories since it was for testing (specifically /home/centos/) and had set these environment variables under /home/centos/.bashrc.
Not an expert on linux, here the export statements are being executed under a user account but the keycloak is being started with sudo so I’m guessing it’s scoping issue that the process isn’t able to discover the exported variables.