Which is the version of JMS Multisites are you using ?
Do you runing in english or in another language ?

This message happen when you try create a slave site for which the "new" DB is not present and that JMS try creating it or when the MySQL user provide does not exists or does not have privilege. In this case, JMS Multisites try creating the users with the parameter provided or grant the existing user with more privilege to access the DB.

The error message is return when all these scenario failled.
At least in english version, you should have a more explicit message.
I suspect that a translation is missing for another language.