您要在调用auth.createUserWithEmailAndPassword之前检查条件.

You want to check your conditions before calling auth.createUserWithEmailAndPassword.

您的if语句上还缺少{，因此它没有达到您的期望.应该看起来像这样:

You've also got a missing { on your if statement, so its not doing what you expect. Should look something like this:

private void signUpUser(String email, final String password) {
   if (password.length() < 6) {
     snackbar = Snackbar.make(activity_criar_conta, "Your password must have at least 6 characters.", Snackbar.LENGTH_SHORT);
     snackbar.show();
     return;
   }
   String pass2 = etConfirmarSenha.getText().toString();
   if (!password.equals(pass2)) {
    snackbar = Snackbar.make(activity_criar_conta, "Both password fields must be identical", Snackbar.LENGTH_SHORT);
    snackbar.show();
    return;
   } 
   auth.createUserWithEmailAndPassword(email, password)
            .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                @Override
                public void onComplete(@NonNull Task<AuthResult> task) {
                    if (task.isSuccessful()) {
                        String emailuser = etCriarEmail.getText().toString();
                        snackbar = Snackbar.make(activity_criar_conta, "Your account was created with sucess: "+emailuser,Snackbar.LENGTH_SHORT);
                        snackbar.show();