You may want to read this,

https://www.digitalocean.com/community/questions/installing-plesk-for-reseller-hosting

Thanks..

After installing plesk I get an error message .

The error is that it can not resolve hostname .

After logging in with " sudo nano /etc/hosts " what should I change ?

I find two lines

127.0.0.1 localhost
127.0.1.1 SERVER SERVER

do it like this..

1.2.3.4.5.6.7 mydomain.com

Do you need a domain name to set up Plesk?