When you uninstall System Center Configuration Manager 2012 it could potentially leave behind some components on your SQL server. You will need to ensure these are removed before you can start the install of a fresh System Center Configuration Manager 2012 or you can run into issues.
Here is the list of what to check:
SCCM 2012 Database:
- Delete your old SCCM 2012 DB from the SQL server.
SCCM 2012 Local User and Groups on SQL Server:
- ConfigMgr Remote Control Users
Registry Key on SQL server:
SCCM 2012 Service on SQL Server: