I went to here to work through the R-keras example:
https://tensorflow.rstudio.com/guide/keras/
There is a command 'predict_classes' that is now deprecated in keras.

You should keep your help documentation reasonably current.
The way to get it to work is:
model %>% predict(x_test) %>% k_argmax()
For discussions related to modeling, machine learning and deep learning. Related packages include caret, modelr, yardstick, rsample, parsnip, tensorflow, keras, cloudml, and tfestimators.