So the root domain (sometimes called the naked domain) is your domain without the www part (called a subdomain). In your case: crossingthethreshold.net.
I glanced at the DNS records for your root domain, and it looks like it lacks the A record pointing to Micro.blog’s IP address. Probably this is what causes the error message you see.
Try following the instructions in Custom domains. Especially this part:
If you want to use a root domain such as
yourdomain.com, create an “A” record wherever your domain is registered and use the IP address value: “104.200.22.214”.