Hi,
It sounds like your database is not an empty database, it already contains some User Defined Functions that mojoPortal is trying to add, so it throws this error.
You can either delete the existing user defined functions for Persian dates or edit the mojoPortal script to remove the sql statements for those functions if they already exist in your database.
The script is at:
/Setup/applications/mojoportal-core/SchemaInstallScripts/mssql/2.3.4.8.config
you can find the Persian date functions by searching for the word Persian
Hope that helps,
Joe