Help please! I am unable to download Mojave as I can't install it to either of my disks.

I am attempting to install Mojave on my 2016 Macbook Pro, but I can't choose either of my disks to install it on. I want to install it on Macintosh HD, but when I click on that drive, I get a message saying that this disk is used for Time Machine backups. I have never used this disk for backups - all backups are stored on my external hard drive.

Why is this happening, and how can I fix it?

Thankyou