You must delete the app access authorisation in the Google account.
To do this, connect to the Google account in question: https://myaccount.google.com.
Select your photo/avatar in the top right then “Manage your Google account” and lastly “Security”. Select “Connect using Google”.
From the displayed list, select the app in question. These may be called “Assimil”, “Bookari” or “Mantano”, then click on “Delete access”.
Once this is done, you should once again be able to select an account to connect to in order to start the app.