If you are using special characters unique to Unicode in your domain name, for example accents such as "è" or "ä" then you will receive the below error:
The specified value "yöur-domaín.com" is not a fully qualified domain name
In order to add these domains to your Vercel projects, you can use Punycode. Punycode is a way of representing Unicode with the limited ASCII character subset that is supported for domains.
For example, the domain in the above image would be represented as "xn--mywbsite-30a.com"
Once you have the Punycode representation for your domain you can follow our guide here and add this in place of the domain name.
You can find a number of Punycode converters online, such as punycoder.com, which you can use to find the ASCII representation of your domain. Then use the converted domain name to successfully add it to Vercel.