AC1200 EX6150 won't set up as extender only access point

I'm having a problem with extender configuration.

Full disclosure, I found this in a free stuff box outside of the old peoples home next door, either way, it appears to be in full working order. After factory reset, though, I can't set it up as an extender. In 'extender setup' mode, I click through and I have the choice between setting up as an access point or as an extender. I choose extender, but at that point the set up program seems to keep looping. I push the button to extender and click continue, and it takes me back to the page where it asks if i want to use smart set up. I click yes. It says 'you have set to extender', i click continue, it loops me back to the quesiton of whether I want to use smart setup. it goes on and on and on. I'm not getting anywhere.

I have no problem setting it up as an access point, so why can't i set it up as an extender?