I've been meaning to write this up for the Sage 100 Routine group but just haven't found the time. Here's what I've found with 2 Premium to Premium upgrades I've done to the 2015 version (1 was from 4.50 and the other 2013). I create a case with Sage so at least one of their Analysts knows about this (Dan Riddle, nice guy and pretty sharp). I'm not sure if this is exactly what you're seeing, but it might be. I'm assuming when you say you have a ""Sage 100 2014 SQL Premium installation issue"" you are migrating from 2014 Premium to 2015 Premium. If not then disregard the rest of this :-)
What is happening is that the 2015 Migration tool is looking on the old SQL server and making backups of the MAS_SYSTEM and MAS_XXX DBs. I'm assuming it does that successfully but haven't verified. Then it is trying to restore those .bak files into the new SQL server so it can use them to migrate the data and populate the new DBs. Unfortunately it doesn't seem to know where to look. It looks in the default path for the Databases in the new SQL Server.
The bottom line here is that to resolve I had to:
1. Make MAS_SYSTEM.bak and MAS_XXX.bak files from the source SQL Server (right click - tasks - backup).
2. Copy those into the default path for your target SQL Server (in my case it was C:\Program Files\Microsoft SQL Server\MSSQL11\MSSQL\DATA)
3. Run the migration utility