Braintree iOS v4 SDK sandbox is not working properly

Question

Previously I was using Braintree iOS v3 SDK but it seems not working anymore so I have changed to iOS v4 SDK.

The issue that I am facing here is when I click any of the buttons, there is no response at all. What might be the problem? Did I miss out something in the setup? I have followed the exact same steps provided by Braintree.

The link of docs: https://developers.braintreepayments.com/start/hello-client/ios/v4#present-drop-in-ui

My code

- (void)viewDidLoad {
[super viewDidLoad];
    self.clientToken = @"token"; //token
}

- (void)showDropIn:(NSString *)clientTokenOrTokenizationKey {
BTDropInRequest *request = [[BTDropInRequest alloc] init];
BTDropInController *dropIn = [[BTDropInController alloc] initWithAuthorization:clientTokenOrTokenizationKey request:request handler:^(BTDropInController * _Nonnull controller, BTDropInResult * _Nullable result, NSError * _Nullable error) {

    if (error != nil) {
        NSLog(@"ERROR");
    } else if (result.cancelled) {
        NSLog(@"CANCELLED");
        [self dismissViewControllerAnimated:YES completion:nil];
    } else {
        result.paymentOptionType = BTUIKPaymentOptionTypePayPal;
    }
}];
    [self presentViewController:dropIn animated:YES completion:nil];
}

- (IBAction)callBrainTree:(id)sender
{
    [self showDropIn:self.clientToken];
}

Any advice is much appreciated.