You must use a desktop UI testing solution to automate Chrome extensions. A pure web testing solution like Selenium can not do automate browser addons, as it has no access to them.
Sikuli is a good option for the automation. Screen-scraping is available if you add Tesseract.
UI.Vision is another good tool and it includes built-in screen scraping (if you ...
Like Niels said above, it could be the timing issue. You might wanna wait until the element exists in the page and then do the stuff. Like you can make a function called WaitUntilElementIsVisible, and call that function before you find any element in the page. Will be easier for you
The code looks like it could work, but maybe the page loads slow and you are trying to interact with an element that does not exist yet. Be sure you wait for the first element to be ready. I added a wait to your example.
IWebDriver driver = new ChromeDriver(service, options);
var wait = new ...