The problem:

I alway got "We couldn't verify your site" from Google Adsense, though I have put the code between and I can see it by view the source code of the page with Chrome.

Solution:

I added another tunel tianlang.tech on the Cloudflare.
I added 'www.tianlang.tech' before and we can't browse it by 'https://tianlang.tech' without the new one.

Cause Root:

It seems the Google Adsense crawls the site by 'https://tianlang.tech' other than 'https://www.tianlang.tech'.
So that we need to make sure the web site can be browsed by 'https://tianlang.tech'.
The website was deployed on my local computer with no public IP, and I use Cloudflare tunel to public it. If you have a public IP, you can just add a new DNS record to allow it to be accessed without a subdomain.