subscription not added

I'm trying to set up membership for my site.

I'm currently having trouble adding a new subscription plan.

I filled out the subscription name, description, price text, then drop a new level into under "Starting Point". After that I specify the Mode, period and price.

It seems pretty straight forward, but when I click the "Add" button, I'm taken back to the list of subscription plans with the message "subscription not added"

What did I do wrong?