If your DNS provider is Wix, domain authentication won't be able to be completed. The issue revolves around how Wix handles MX records for subdomains: Wix does not allow MX records to be published for a subdomain, which is required to complete the FUB Domain Authentication steps.
If that changes in the future we will update this help article with the steps to complete domain authentication.
As it stands right now here are your options moving forward.
1) Continue using Wix for DNS management and send Batch Emails through FUB to smaller audiences for better deliverability. If you use Google Workspace for email you can send to up to 1000 recipients in one day through FUB Batch Email and it will send from your connected email servers with good deliverability. If you use Microsoft 365 for email then you can send to up to 2500 recipients in one day with good deliverability.
2) If you want to have the option to send to up to 10K recipients per day, domain authentication would need to be completed. In this case, we recommend contacting GoDaddy or CloudFlare (two highly reputable DNS providers that allow FUB domain authentication to be completed) and pointing your nameservers to one of those systems which means you would manage DNS records for your domain in their system.
If you choose option 2 above, you can follow the steps in this article to complete Domain Authentication.