If the automatic Crawler validation doesn't work after having followed all the steps outlined in the doc Verify your domains and you get the warning message: Code was not found in " https://your-domain.com/robots.txt ", you need to check that the robots.txt is correct and make sure that it hasn't any indentations in front of each line (no spaces or tabs). In case it does, you'll need to remove whitespace.
If the issue persists, contact Support