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In case anyone else is having the same issues: “@” just means that you need to enter the DNS record under “Domain Records (required)” section at the top (as opposed to the “Subdomain Records (optional)” section at the bottom). The “@” does not need to be entered at all. instructions on how to set up your DNS settings:
How do I set up DNS for my domain? https://www.dynadot.com/community/help/question/set-up-DNS
How do I enter a TXT record for my domain? https://www.dynadot.com/community/help/question/create-TXT-record
How do I set up my MX records so I can use my domain email if I'm using DNS? https://www.dynadot.com/community/help/question/MX-DNS-set-up