Membership plugin associated role only show one select option which is subscriber role

I recently setup a wordpress multisite and i downloaded and install the membership plugin in which I only activated the plugin in per site basis.

On the main blog, I configured the 2 access level or membership level in the following:

1. Trial Membership

It is only for 30 days period and I set the default role is subscriber in the associated role section.

2. Full Membership

It is a 1 year subscription but the problem is when I want to set the default role to author or contribution, it gives me only subscriber role only. I tried to check my trial membership's associated role section and also show one option only which is the subscriber role only.

Is it a bug? or I just missing something?

Please help.