What helped for me were the following steps:

1. Add ‘DISPLAY=:1’ to my crontab
2. Set the correct shell in the crontab (I use ‘zsh’)
3. Source the environment variables since crontab does not do this by default
4. Use absolute path to the chromedriver

TLDR

1. The required modifications to your crontab would be:

   SHELL=/bin/zsh
   05 * * * * export DISPLAY=:<displayNumber> && source /home/<username>/.zshrc && cd <absoluteExecutableDirectory> && ./<pythonFile> >> log.log 2>&1
   

2. Use the following line to initialize the selenium chrome driver:

   driver = webdriver.Chrome(<absoluteDriverPath>,...)
   

Replace everything within the angled brackets with their respective values.

1. Set display

To find out which display to add to your crontab, use:

    env | grep 'DISPLAY'

Then add this piece to your crontab command:

    export DISPLAY=:1

2. Set shell

• If you have a non-default shell*, then set the shell.
  Find out the location of your shell with one of the two commands

  which bash
  which zsh
  

  Then set the shell to the response of the previous command (in your crontab):

  SHELL=/bin/zsh
  

3. Source the environment variables

Add one of the following piece to your crontab command, depending on wether you use bash or zsh:

    source /home/<username>/.zshrc
    source /home/<username>/.bashrc

4. Use an absolute path for your chromedriver:

When initializing the driver use the following line where points to the selenium chrome driver.

    driver = webdriver.Chrome(<absoluteDriverPath>,options=options)

Other

    >> log.log 2>&1

Means that all output is written to a file (This allows for easier debugging the crontabs).

On ubuntu 18.04, Python 3.6.9:

It’s on my main workstation, so I always have a logged in X session going.

my selenium invocation:

#!/usr/bin/python3
from selenium import webdriver
from selenium.webdriver.common.keys import Keys
from selenium.webdriver.common.by import By
driver = webdriver.Firefox()
location = 'http://example.com'
driver.get(location)

what I put in crontab:

33 14 * * * DISPLAY=:0 lxterminal --working-directory=/home/user/Documents/ -e /home/user/bin/get.stats.by.zip.py > /home/user/gbzp.log 2>&1

lxterminal is a relatively simple terminal program that I tend to use for most terminal stuff, instead of, say gnome-terminal, installed with apt-get.

I don’t know if I’m late but i think my solution is going to help a lot of people.I have found the solution after trying for 1 1/2 hrs. The solution steps are:

1. Open Terminal and run the command:

echo $DISPLAY

Let the output of this command is: :0

2. Open the crontab for editing:

crontab -e

3. Inside the crontab, append these lines:

     DISPLAY=:0 
     ## If output of "echo $DISPLAY" is: ":1", then change the above line to: "DISPLAY=:1" (without quotes)


     ## if running the python file every 2 minutes:

     # If firefox is used for selenium automation:

     */2 * * * * export PATH=$PATH:path_to_python_executable_folder:geckodriver_folder_path_for_firefox; python path_to_your_python_script.py

     # If chrome is used for selenium automation:

     */2 * * * * export PATH=$PATH:path_to_python_executable_folder:chromedriver_folder_path; python path_to_your_python_script.py