you should use double curly brace, instead of

      Payload: '{"Name": "${env.BRANCH_NAME}" }'

use

      Payload: '{"Name": "${{env.BRANCH_NAME}" }}'

you don't need an extra step to build the payload

UPDATE

Seems the issue is the env definition in the env section: moving the var definition in a dedicated step will solve the problem. Of course this solution is similar to build the payload in a dedicated step. See this PR https://github.com/mbiagetti/github-action-poc/pull/4/files

How to use an event.json file locally with nektos/act from their documentation they describe structure like:

# File: /path/to/event.json
{
  "pull_request": {
    "head": {
      "ref": "sample-head-ref"
    },
    "base": {
      "ref": "sample-base-ref"
    }
  }
}

Or follow steps in answer above to have github output the action event details and paste into this event.json file

Then to use the event file locally you can add the option --eventpath '/path/to/event.json' to your run act command, example:

act -W '.github/workflows/run_this_workflow.yml' -s GITHUB_TOKEN="$(gh auth token)" --eventpath '/path/to/event.json'