SendBoard works by having an Automated Trello User (bot) add cards and comments for emails you receive. In some cases, we try to add a bot to a Trello board, but run into a problem.
The most likely reason you'd have this issue, is a setting available to Trello Business Class Teams called "Inviting Guests to Boards". If this is set to "Only Team Members", we will be unable to add the bot as a guest user.
You'd need to change this setting to "Anybody" and then try again to complete the SendBoard set up process.
Once the bot has been added, you can change the setting back to "Only Team Members".