Membership subscription upgrades: "Upgrades not available"

I have the latest Membership Plugin installed and enabled. I login and go to the “renewal” page which has the [renewform]. The subscription upgrade buttons are marked “Upgrades not available”. Why?

I am using a Paypal and 2checout gateway

I have set the Renewal period limit : 1 day

Upgrades period limit :: 1 day

in the configuration setting