See also Nohost domain recovery - Suppression de domaine en nohost.me et noho.st
In order to get someone (probably @Moul 
) to delete the DNS record corresponding to your preconfigured domain(s)
yourpreconfigureddomain.noho.st
or
yourpreconfigureddomain.nohost.me
OR
Start to enter a dummy domain, supposedly being already configured: yourpreconfigureddomain.dummy, then proceed to installation.
Later on, change the domain name to your domain previously preconfigured by yunohost, then set it as default domain, and finally delete yourpreconfigureddomain.dummy