The startup disk cannot be partitioned or restored to a single partition.

Hello everyone. I want to just run Windows 7 on my MacBook Pro but it is not letting me do it. I am getting the error "The startup disk cannot be partitioned or restored to a single partition." I don't understand why it is giving me this error. I only have one partition on my MacBook Pro, and that is the OS. And yes, I did delete the Recovery Partition as it does not show up in the Drive Selection menu (holding option on boot). I am just hoping somebody could help me with this. 😟

ℹ Specs

Mid 2010 13in MacBook Pro

macOS High Sierra 10.13.1

2.4 GHz Intel Core 2 Duo

4GB DDR3 RAM

NVIDIA GeForce 320M | 256MB VRAM

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I'm trying to install Windows 7.