Why this error occurred

In single package mode, there cannot be multiple packages in your repository. Therefore, declaring a task in the turbo.json with a specified package name is not permitted.

{
  "tasks": {
    "app#build": {
      "cache": true
    }
  }
}

Solution

Remove the package name from the task declaration.

{
  "tasks": {
    "build": {
      "cache": true
    }
  }
}

Alternatively, if you would like to have multiple packages, you can specify the workspaces in your repository.