You tried to accept an invitation to join a board, but the organization that owns that board doesn't allow files to be shared with outsiders any more.
This happens sometimes: companies may decide that they don't want their Google Docs or Box files shared with anyone who is not an employee.
And it looks this decision got made in the short time between when someone decided to invite you to join their board, and when you tried to accept the invitation.
Sorry: this is a weird situation and there's not much we can do in this situation. You need to talk to the person who turned off access for you...