Attempting to get a key where value is equal to in Realtime Database

Question

I have the following structure:

attendanceOptions: 
  uid: "[email protected]",
  uid: "[email protected]",

I'm attempting to get the key and value where value is... In example get where [email protected]

Here is my attempt:

security rules:

{
  "rules": {
    ".read": true,
    ".write": true,
    "organization": {
      "$uid": {
        "attendanceOptions": {
          ".indexOn": ["$uid"]
        },
        "members": {
          ".indexOn": ["mStatus"]
        }
      }
    }
  }
}

The error I get is

Error saving rules - Line 9: Invalid key: $uid. Index must be either .value or declared on a valid path

Obviously I'm doing something wrong. How can I accomplish this?