ValueError: x is required to have ndim 1 but has ndim 3

Question

I loaded LSTM & CONV2D code and plot it into a ACF graph . But it returned this error (x is my dataframe variable):

ValueError: x is required to have ndim 1 but has ndim 3

Here is the (CONV2D) code:

(X_train,y_train) , (X_test,y_test)=mnist.load_data()
#reshaping data

X_train = X_train.reshape((X_train.shape[0], X_train.shape[1], X_train.shape[2], 1))
X_test = X_test.reshape((X_test.shape[0],X_test.shape[1],X_test.shape[2],1))

X_train = X_train.reshape((X_train.shape[0], X_train.shape[1], X_train.shape[2], 1))
X_test = X_test.reshape((X_test.shape[0],X_test.shape[1],X_test.shape[2],1)) 
 

#defining model
model=Sequential()
#adding convolution layer
model.add(Conv2D(32,(3,3),activation='relu',input_shape=(28,28,1)))
#adding pooling layer
model.add(MaxPool2D(2,2))
#adding fully connected layer
model.add(Flatten())
model.add(Dense(100,activation='relu'))
#adding output layer
model.add(Dense(10,activation='softmax'))
#compiling the model
model.compile(loss='sparse_categorical_crossentropy',optimizer='adam',metrics=['accuracy'])
#fitting the model
model.fit(X_train,y_train,epochs=5)

#evaluting the model
model.evaluate(X_test,y_test)

acf_plot = plot_acf(X_train, lags=20)
acf_plot = plot_acf(X_test, lags=20)