Some of the influencers may be missing an email from their Upfluence profile. If an influencer does not have an email address available, you will not be able to move them into a mailing campaign.
You can find out if an influencer has an email address by looking at their profile. If they have a chat sign it means they have an email:
You can also click into their profile and go to the "Influencer" tab to check.
One good way to avoid selecting influencers without an email address available is by selecting "Has an Email" during your search: