Thank you @LeonidS I got it working! But I think I found a bug, maybe?

I had been using the search function from the permissions app in studio - typing in "Channels" to find the permissions. I tend to default to typing since I'm super fast at it...

If I do that, this is what I see:

I did this yesterday. I thought "Wait I don't remember turning all channel permissions off for Site Admins, that's super weird, but oh well!", then I turned them all on, and went to do my edits, and creating the channels did not work.

Then today I did the same thing again, and saw all the permissions turned off and was like "waaait a minute I KNOW I turned these on yesterday..."

So - searching appears to not work unless a module is selected - the search returns a list of things, but the permission setting does not match. If I click the sliders to enable the permissions, they appear to turn on, but don't really turn on.

But if I instead select the module from the menu, then I see the real permissions for the selected membership level:

And from this screen, turning on "Create Channel Auto" worked, and then creating new channels worked. So I'm all set now. :-)

Haha low priority bug if it is a bug since the workaround is very easy, but figured I would share. Thank you so much for your help!