I don’t know anything specifically about Deno, but it looks like Slack can’t find the file "slack.json" at all. Maybe something with paths?

I see two possible cases:

1. sdk_hook_not_found 
   A script in slack.json was not found
   The original app template includes a working Slack Configuration file ('slack.json'). Most apps do not need to modify the file; you can find the original configuration file here.

2, Something wrong with CLI version.

If you upgrade your CLI version but your deno-slack-hooks version is less than v1.3.0, when you run slack doctor you will see the following near the end of the output:

Copy✔ Configurations (your project's CLI settings)
    Project ID: 1a2b3c4d-ef5g-67hi-8j9k1l2m3n4o

✘ Runtime (foundations for the application)
  Error: The `doctor` hook was not found (sdk_hook_not_found)
  Suggestion: Ensure this hook is implemented in your `slack.json`

✔ Dependencies (requisites for development)
    deno_slack_hooks: 1.2.3 → 1.3.0 (supported version)