Hello @user509722 ,
Trialers can have up to 100 sendouts. This isn't email campaigns, but how many times contacts have been sent to. This applies whether you've sent one campaign to 100 contacts, 5 campaigns to 20 contacts each time, 10 campaigns to 10 contacts each time, or some other combination that results in 100 sendouts. If the amount you'd be sending to would go over 100, that error messaging will show up.
Once you have a paid account, you'll be able to send as many email campaigns to your contacts as desired, with respect to basic anti-spam best practices of course.