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thanks for your reply.
This is written on the google site: Important: If you use CAA Records on your custom domain, add a record for letsencrypt.org, or Blogger won't create or renew your SSL certificate.
That is a very specific point about CAA. If you don't use CAA, that's not relevant.
Luckily, the guide from which you copy/pasted that specific sentence, does include a lot more information and steps to perform. At least, when I searched on "blogger.com let's encrypt" through Google, I came across Turn on HTTPS for your blog - Blogger Help which includes the "Important" statement and the how-to encrypt your site through Blogger.com.
Please go through all the steps on that guide. If it somehow doesn't succeed, please tell us as specific and detailed as possible, preferably with screenshots of the error and steps you did before the issue presented itself.