The captcha prompt usually appears if you have initiated a lot of search queries within a short period of time. You need not worry about this since Google is just trying to tell humans and search bots apart. You can safely answer the captcha to proceed with the search.
If you can't do keyword research because you're getting too many captchas, the best option is to use a captcha-resolving service. Setting the timer delay value to 7 or more seconds should also help.
For more suggestions, see this article.