I do not see the Fracture mode when I click on the modes, and I cannot place a chaos solver actor.
I have all the chaos plugins enabled by default, and it’s a fresh install of the 4.26 Preview, what am I missing?
I am in the same situation as you
The chaos system has been postponed, due to not being stable and production ready. So to utilize Chaos you have to build from source. 
If you compile 4.26 p5 the fracture will be under modes. Chaos seems to OK, but a little slow.