I am getting the above error while creating Visualforce page through code provided by Salesforce in Trailhead for Einstein.This was downlaoded from Github provided through link provided by Salesforce. Not sure what the error is and not able to resolve. Any help will be great. Stuck in the trailhead. 
The stage is for Trailhead Module : Get Smart with Salesforce Einstein  Quick Start: Einstein Image Classification: Create the Visualforce Page
 
<apex:page Controller="VisionController">
  <apex:form >
  <apex:pageBlock >
      <apex:image url="https://einstein.ai/images/generalimage.jpg">
      </apex:image>
      <br/>
      <apex:repeat value="{!AccessToken}" var="accessToken">
          Access Token:<apex:outputText value="{!accessToken}" /><br/>
    </apex:repeat>
      <br/>
      <apex:repeat value="{!callVisionUrl}" var="prediction">
          <apex:outputText value="{!prediction.label}" />:<apex:outputText value="{!prediction.probability}" /><br/>
      </apex:repeat>
  </apex:pageBlock>
<!--  <apex:pageBlock > -->
<!--      <apex:repeat value="{!callVisionContent}" var="prediction"> -->
<!--          <apex:outputText value="{!prediction.label}" />:<apex:outputText value="{!prediction.probability}" /><br/> -->
<!--    </apex:repeat> -->
<!--  </apex:pageBlock> -->
  </apex:form>
</apex:page>