This error message effectively means that the connection to the company you are trying to activate is already active in the account you are in.
So you don't have to do anything anymore and you will find this connection in the "Connections" section of your Doccle account.
Attention:
Not all Doccle senders offer an archive of documents created prior to the activation of their connection. In that case, you will not yet find any documents from the company involved and you will receive your first document from the first document created after the date of activation of the connection.