If the email campaign was created as part of a center, only constituents who have opted in to that center are eligible to receive the email.  For example, if a campaign was created in Center A, but the target audience group is in Center B, then only those constituents who have opted in to both centers would be eligible to receive the mailing.  In this case, anyone who has not opted in to Center A would be removed by the Center Opt-In filter. 

To fix this, you can either change the security category of the campaign to General or select a target audience group that consists of members from the same center as the email campaign.

It should also be noted that members must be opted into a center, or else this error may occur.  If you have a welcome series and members are not uploaded or assigned a center upon creation, they may be opted out by this filter if you are using a center specific email group and campaign.