Thank you for the post.
This error usually indicates one of two issues:
1. Migration Assistant brought over an FM Server user account from a previous FileMaker Server installation.
2. One of the accounts on this computer is already named FM Server.
The resolution to both of these is the same:
1. Log into a user NOT called FM Server.
2. Run the following command in Terminal:
sudo dscl . -delete /Users/fmserver
Afterwards, reinstall FileMaker Server 13 and the user will be created during the installation.