Hello @TomG64908 ,
Are you trying to schedule a social share for your last email? You'll need to connect a Facebook Business page to the account first. Once you have it connected, that grayed out box can be clicked so you can select which business account(s) you want to have us post the email webpage version and your message to.
You may also want to look into using the social tab at the top of your screen for a more robust way of posting to social. You just connect your social media business page, write your message and paste the email's share link, and then schedule or post immediately like you would a normal social media post.