Regardless of the process used, CAPTCHAs may occasionally interfere with mass enrolling Chromebooks. In our experience across many hundreds of thousands of automated enrollments, while the CAPTCHA response may be randomly triggered, once the CAPTCHA has been manually entered a few times, it rarely appears again during the remainder of the enrollment.
If it becomes a nuisance, it is possible to white-list your IP address to disable the CAPTCHA. This can be requested via your Google technical support channel with your support PIN.