The error's a 'file not found', so the obvious question is, is the file there?
Double-check that path please, because it doesn't look correct
C:\Program Files\Microsoft SQL Server\MSSQL13.MSSQLSERVER\MSSQL\Galactic.mdf
With default installation settings, that should be C:\Program Files\Microsoft SQL Server\MSSQL13.MSSQLSERVER\MSSQL\Data\Galactic.mdf
Similar for the log file (whose name also looks wrong, Galactic_log.ldf would be the default name)
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability