This error is caused by using code written for the ArduinoJson library 5.x.x when you have have ArduinoJson library 6.x.x installed. The easiest solution is to use the version of ArduinoJson the code was written for:
- Sketch > Include Library > Manage Libraries
- Wait for the download to finish
- In the "Filter your search..." field, type "arduinojson"
- Press Enter.
- From the search results, click on "ArduinoJson by Benoit Blanchon"
- From the dropdown "Select version" menu, select "5.13.5".
- Click the "Install" button.
- Wait for the installation to finish.
- Click the "Close" button.
Your code should now compile without any error.