JsonRpcError when sending transaction using MetaMask

[devsideal]

When I'm calling a custom contract method to send transaction through metamask app, getting below error Unhandled Exception: JsonRpcError(code: 5000, message: Internal JSON-RPC error.)

STR I have three step process like:

1. Connect (connect to Metamask wallet app) : Working
2. Approve (approve) : Not Working
3. Complete (minting):

Step-1 is working fine when i click on Connect button its open the Metamask and i successfully received chain id and account address. But when i click on Approve button its open the Metamask app & show the Modal sheet with Approve and Reject buttons, when click on Approve It shows Internal JSON-RPC error alert as showing in screenshot:

My Code is:

Future<dynamic> callApprove(String conAddressHex, int amount, BuildContext context) async {
    String transactionId = "";
    try {
      /// LOAD SMART CONTRACTOR
      var contract = await _loadContractERC20();

      /// MAKE TRANSACTION USING web3dart
      Transaction transaction = Transaction.callContract(
        from: EthereumAddress.fromHex(_accountAddress!),
        contract: contract,
        function: contract.function(_methodApprove),
        parameters: [
          EthereumAddress.fromHex(conAddressHex),
          BigInt.from(amount) * BigInt.from(1000000000000000000)
        ],
      );

      /// MAKE ETHEREUM TRANSACTION USING THE walletconnect_flutter_v2
      EthereumTransaction ethereumTransaction = EthereumTransaction(
        from: _accountAddress ?? "",
        to: conAddressHex,
        value: '0x2386F26FC10000', // 0.01 Ether (in wei)
        data: hex.encode(List<int>.from(transaction.data!)), /// ENCODE TRANSACTION USING convert LIB
      );

      /// OPEN METAMASK
      await launchUrl(Uri.parse("$wcUriPrefix$_encodedWcUrl"),
          mode: LaunchMode.externalNonBrowserApplication);

      ///  REQUEST TO WALLET FOR TRANSACTION USING walletconnect_flutter_v2
      transactionId = await _web3App?.request(
        topic: _sessionData?.topic ?? "",
        chainId: kFullChainId.toString(),
        request: SessionRequestParams(
          method: "eth_sendTransaction",
          params: [ethereumTransaction.toJson()],
        ),
      );
    } on Exception catch (_, e) {
      _log("Error:  $e");
    }
    _log("transactionId:  $transactionId");
    return transactionId;
  }

Anyone, can you help me what's going wrong?