It sounds like the whole SSD is devoted to one partition with Windows on it, which I would expect. A dual boot on one drive requires shrinking that existing partition so that another partition can be created on the SSD for the second operating system. Simply put, there's no room there for Solus. Or Manjaro, Ubuntu ...
This is somewhat dangerous, as it's quite possible to damage your Windows installation if you make a mistake. If you go that route, make sure you have a good backup, so you can restore Windows if anything goes wrong.
If you'd like to consider a different approach, think about installing VirtualBox (it's free) on your Windows partition, and creating a virtual machine (VM) for Solus. It's an easy application to use, and there are lots of advantages.
You can use Windows and the VMs at the same time, without saving all your work and booting to the other system. You're not limited to running just two systems. I typically have three or four editions of Solus on my Windows laptop, running in VMs at the same time. Putting them into different workspaces in Windows* allows you to easily choose one to work with just by changing to the workspace where it's running.
A VM can easily be removed and its resources freed if you decide you don't need it anymore. The VirtualBox manager makes that easy. So damaging a VM is nowhere near the disaster that damaging Windows would be. And since you can have all the VMs that you want, you can feel free to try out new versions of other distributions. It takes only a few minutes to create a VM for the purpose, and if you decide not to continue with that distro, you can just remove the VM.
VirtualBox makes it easy to create snapshots of a VM, so you can easily return to any prior state. Thus, backups are trivial. If an upgrade breaks anything, or you run into some malware, you can just return to the previous snapshot and keep working.
*There's a workplace switcher for Windows that's very Linux-like. It's called VirtuaWin, and it's free. It's been around for decades, and it's very versatile and reliable. I configure VirtuaWin on my Windows laptop with 8 desktops, 4 of which I use for Windows to keep applications from piling on top of one another, and 4 of which I use for various VMs. It's the one tool I don't think I could use a Windows computer without!
