That not how the tool works.
As you can see in your sensor, the actual template section is just: {{ value_json.timestamp | int}}
Everything else is going to be ignored. So value_json is empty at that point.
For example:
This will do nothing
Just text
template:
switch:
- name: "this tool does not care"
state: {{ states('switch.relay_1_relay') }}
Will output:
Result type: string
This will do nothing
Just text
template:
switch:
- name: "this tool does not care"
state: off
This template listens for the following state changed events:
Entity: switch.relay_1_relay
So you need to use an active entity or provide the data yourself like koying did.
Thats how you test out your template by providing the data.
Does anything show up in your error logs?