I installed multi-domain and it suggests the following
Please copy the content of sunrise.php into /home/user/domains/our.domain/public_html/wp-content/sunrise.php and uncomment the SUNRISE setting in the /home/user/domains/our.domain/public_html/wp-content/wp-config.php file.
Please uncomment the line //define( 'SUNRISE', 'on' ); in the /home/click/domains/smartclick.site/public_html/wp-config.php file.
I had a look into our wp-config file but this line did not exist in there.
Therefore I would recommend either to write this line into the wp-config.php always when it gets installed and it isn't already available in that file
changing the wording of the message to:
Please copy the line //define( 'SUNRISE', 'on' ); in the /home/click/domains/smartclick.site/public_html/wp-config.php file and uncomment it, if it isn't already present and active in your wp-config.php file.
Than the error text would be also a help for those who can't find that line and than perhaps give up.
hmm - but the first message is still there even than and the sunrise.php file is already in wp-content.