When the Send email button is grayed out, there are no emails to send due to the following situations: 

There is no one with an expiration date that falls within the range of the membership renewal packages.

When setting up your membership renewal pages, you selected to use Mail when no email or mail preference was specified on the constituent record. If you would like to use email instead, follow these steps to edit your membership template: 
  1. Go to Marketing and Communications > Membership Renewal Efforts
  2. Click the Templates Tab. 
  3. Click your Membership Renewal Template
  4. On the Membership Rules tab, move the email packages so that they appear before their mail package counterpart using the Move up or Move down buttons. (For example, when you finish editing, your rules may appear similar to this: Membership - 1 month before expiration (email), Membership - 1 month before expiration (mail), Membership - Month of expiration (email), Membership - Month of expiration (mail), etc.)
  5. Run your membership renewal processes again and you should see the option to Send email.