I was trying to create a circle of spores druid today and it’s not available to choose. I’ve owned the content since it was released and I’ve made druids of that circle before but today it’s not there. Stars and Wildfire are, along with other Tasha’s content for other classes.

That subclass was originally published in an M:tG sourcebook. On the Home tab of your character builder you have to make sure that the toggle next to “Magic: the Gathering Content” has been enabled (red, to the right).

Even though it’s in Tasha?

Yes, because it was originally published in an M:tG book. That’s just how it works here.