I would try "Start Over" to see if it is just the program messing up.
The first time I tried this I got the same results. It wouldn't let me add a material.
Then SW told me my material had no structural properties, even though they are clearly there. It put a green check next to Material as though it had added it, but nothing changed.
Then I clicked "Start Over" and tried again and it worked. I have no idea why.
I tried again with "Start Over" and couldn't get it to add a fixture, which was greyed out.
I had to shut down and restart SolidWorks to get it to work correctly.
thank you very much