@Jonas thanks for the suggestion. This is my updated code and test ran successfully.

public async Task CreateTransaction_OnSuccess_ReturnStatusCode200()
{
    //Arrange
    var mockTransactionService = new Mock<ITransactionService>();
    var transactionRequest = TransactionFixture.CreateTransaction();
    var transactionResponse = new TransactionResDto { Status = true };
    mockTransactionService.Setup(service => service.CreateTransactionAsync(It.IsAny<TransactionReqDto>()))
                        .ReturnsAsync(transactionResponse);
    var mockTransactionController = new TransactionController(mockTransactionService.Object);
    //Act
    var result = (OkObjectResult) await mockTransactionController.CreateTransaction(TransactionFixture.CreateTransaction());
    //Assert
    result.Should().BeOfType<OkObjectResult>();
}