GRUB / os-prober doesn't see Windows 8.1 partition
You should use boot-repair. It will in most cases be able to help fix the problem.
Boot-Repair is a simple tool to repair frequent boot issues you may encounter in Ubuntu like when you can't boot Ubuntu after installing Windows or another Linux distribution, or when you can't boot Windows after installing Ubuntu, or when GRUB is not displayed anymore, some upgrade breaks GRUB, etc.