Try with direct payload

.github/workflows/on-fail-json.yaml

@@ -0,0 +1,58 @@
+name: Notify on `main` Failure
+on:
+  workflow_run:
+    workflows: [Failing Workflow]
+    types: [completed]
+    branches: [main]
+
+jobs:
+  on-failure:
+    runs-on: ubuntu-latest
+    if: github.event.workflow_run.conclusion == 'failure' || github.event.workflow_run.conclusion == 'timed_out'
+    steps:
+      - uses: slackapi/slack-github-action@v1.24.0
+        with:
+          payload: |
+{
+  "text": "main branch failure in CI ",
+  "blocks": [
+    {
+      "type": "header",
+      "text": {
+        "type": "plain_text",
+        "text": ":warning: `main` branch failure in CI ",
+        "emoji": true
+      }
+    }, 
+    {
+      "type": "section",
+      "text": {
+        "type": "mrkdwn",
+        "text": "View run details: "
+      },
+      "accessory": {
+        "type": "button",
+        "text": {
+          "type": "plain_text",
+          "text": "Run Details",
+          "emoji": true
+        },
+        "value": "clicked_1",
+        "url": "${{github.server_url}}/${{github.repository}}/actions/runs/${{github.event.workflow_run.id}}",
+        "action_id": "button-action"
+      }
+    },
+    {
+      "type": "context",
+      "elements": [
+        {
+          "type": "mrkdwn",
+          "text": "Linked Repo <${{github.server_url}}/${{github.repository}}|${{github.repository}}> | <${{github.server_url}}/${{github.repository}}/actions/runs/${{github.event.workflow_run.id}}|View Failure>"
+        }
+      ]
+    }
+  ]
+}
+        env:
+          SLACK_WEBHOOK_URL: ${{ secrets.MOSHE_PERSONAL_SLACK_WEBHOOK_URL }}
+          SLACK_WEBHOOK_TYPE: INCOMING_WEBHOOK