• You have ad crawler errors, which can result in lost revenue.
• Crawler: Robots.txt file is unreachable
• We were unable to reach your robots.txt file

Test your robots.txt with the robots.txt Tester

https://www.google.com/webmasters/tools/robots-testing-tool

The issue can be raised from SSL error

Get SSL Fixed or turned off completely.

Check whether you have the ads.txt file uploaded

The ads.txt error is similar. ADSense will check