Connection refused, errno = 111, address = localhost, port = 57838 #0

[fabrice]

If you are using Sandbox, the port should be 4002 not 57838.
See GitHub - algorand/sandbox: Algorand node sandbox

Can you show the code of CreateWalletRequestBuilder()?

Also, the use of kmd is very limited.
Most dApps should use AlgoSigner (AlgoSigner | Algorand Wallet Extension for Chrome | PureStake), MyAlgoConnect (Introducing MyAlgo Connect for DApp Developers | Algorand Developer Portal), or WalletConnect (WalletConnect - Algorand Developer Portal).