WHMCS MU Provisioning doesn't create / activate automatically

I have the WHMCS MU Provisioning plugin/add-on installed and on the backend everything looks like it should, but when a user places an order, the account is not actually created in the WP site. Their order shows as "pending". I have tried a couple different settings, one for the site to be created when the order is placed, and the other when payment is received, but neither sets up the site automatically. The user's details are stored and I can go into the order in WHMCS and click "create" and it will create the account for the user like it's supposed to do automatically, but it's just not happening automatically. I've tried a couple different gateways. When I do manually go in and click "create" under the user's order, I get an error message (unfortunately I didn't get a screenshot of it), I think saying something about the admin user not existing or something like that, but the thing is it works. I get an error, but the process works. It creates the site and everything looks fine. If I see the error again I'll take a screenshot, but my main concern is that the setup isn't happening automatically.