Restore-SPSite : The operation that you are attempting to perform cannot be completed successfully. No content databases in the
web application were
available to store your site collection. The existing content databases may
have reached the maximum number
of site collections, or be
set to read-only, or be offline, or may already contain a copy of this site
collection. Create anoth
er content database for the
Web application and then try the operation again.
1- Open Central Administration.
2- Open Application Management -> Click Manage Content Database
3- Select Your Web Application, Click -> Add Content
4- just create new Content Database under your web application and try to restore again.