Can a Single user use Tally Server 9?

No.

Tally Server 9 is mainly installed to solve the problem of Tally slow or concurrency. Concurrency occurs when there are more than one users in Tally. In a single user Tally environment, Tally Server 9 cannot be installed because there is only one user and hence the issue of concurrency does not arise. This is why Tally Server 9 is not installed in a single user Tally software.