Create a wallet as follows:

```sh
$ bitcoin-cli createwallet "watchonly" true
$ bitcoin-cli -rpcwallet=watchonly getwalletinfo
{
   ...
   "private_keys_enabled": false
}
```

The following should fail, but doesn't:

```sh
$ bitcoin-cli -rpcwallet=watchonly encryptwallet 1234
wallet encrypted; The keypool has been flushed and a new HD seed was generated (if you are using HD). You need to make a new backup.
```

It partially fails silently, because it doesn't actually add a seed. But it does ask for a password when calling `dumpwallet`, which doesn't make sense because we currently don't encrypt anything other than private keys.  
{
    std::shared_ptr<CWallet> const wallet = GetWalletForJSONRPCRequest(request);
    CWallet* const pwallet = wallet.get();
    if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
        return NullUniValue;
    }
    if (request.fHelp || request.params.size() != 0) {
        throw std::runtime_error(
            RPCHelpMan{"walletlock",
                "\nRemoves the wallet encryption key from memory, locking the wallet.\n"
                "After calling this method, you will need to call walletpassphrase again\n"
                "before being able to call any methods which require the wallet to be unlocked.\n",
                {},
                RPCResults{},
                RPCExamples{
            "\nSet the passphrase for 2 minutes to perform a transaction\n"
            + HelpExampleCli("walletpassphrase", "\"my pass phrase\" 120") +
            "\nPerform a send (requires passphrase set)\n"
            + HelpExampleCli("sendtoaddress", "\"1M72Sfpbz1BPpXFHz9m3CdqATR44Jvaydd\" 1.0") +
            "\nClear the passphrase since we are done before 2 minutes is up\n"
            + HelpExampleCli("walletlock", "") +
            "\nAs a JSON-RPC call\n"
            + HelpExampleRpc("walletlock", "")
                },
            }.ToString());
    }
    auto locked_chain = pwallet->chain().lock();