Run terraform apply to populate the output. This only needs to be done once after the output is defined. The apply output should change slightly. At the end you should see this:

$ terraform apply

…

Apply complete! Resources: 0 added, 0 changed, 0 destroyed.

Outputs:

ip = 50.17.232.209

apply highlights the outputs. You can also query the outputs after apply-time using terraform output:

$ terraform output ip

50.17.232.209

This command is useful for scripts to extract outputs.