I am struggling with local sentinel developement as well.
I am using MAC. and Sentinel v0.40.0 and Terraform v1.10.5. My plan file is called `plan.json`

When using this example I always fail

# create file policy.sentinel
vim policy.sentinel

...
sentinel {
  features = {
    apply-all = true
    terraform = true
  }
}

import "plugin" "tfplan/v2" {
  config = {
    "plan_path": "./plan.json"
  }
}
...
:wq!

# Execute file
$ sentinel apply  policy.sentinel

Error parsing policy: policy.sentinel:1:10: expected ';', found '{' (and 2 more errors)

Any suggestions ?