Why can’t I select more than one Twitter while scheduling posts?

You will be allowed to schedule tweets to one Twitter account only (at a time). 

Twitter introduced new guidelines disallowing all third-party apps like SocialPilot to permit users to share the same post to multiple Twitter accounts at the same time. The new guidelines aim to “prevent people from exploiting several accounts for artificially amplifying a message”. You can learn more about guidelines here.

You can create the post again and select another Twitter account.