Many browsers now enforce the use of HTTPS on websites to allow them to be used. If you, like me, want to enable this in Blogger (Setting > Basic > HTTPS - HTTPS availability) for your own domain (e.g. not ending “blogspot.com) but you get the error “bX-2kvjq7” you might not have validated your domain.
In which case try: Settings > Basic > Publishing - Blog Address > Edit.
Then make sure you’ve added both the CNAMEs suggested. If that doesn’t work, click the link in “you can verify your ownership of this domain with a TXT record through the Google Search Console here instead”. After the jump choose your domain name provider from the drop down and add the long string as a TXT record in your DNS configuration as instructed.