Making 2008 Macbook work with new SSD running Sierra
Sierra is not compatible with your 2008 MacBook1

Mac Supported Hardware:


MacBook (Late 2009 or newer)
MacBook Pro (Mid 2010 or newer)
MacBook Air (Late 2010 or newer)
Mac mini (Mid 2010 or newer)
iMac (Late 2009 or newer)
Mac Pro (Mid 2010 or newer)


As to why your machine is coming up as non-bootable is because your Mac cannot read the new Apple File System (APFS) which centers around security and encryption.2  Basically, your Mac's firmware cannot read the drive to boot from it.



1 MacOS Sierra - Technical Specifications

2 What MacOS Sierra's New APFS File System Means to You