There is currently an issue with Google when the Portant Add-on first opens. The first thing that happens is that Google makes sure you have access to that document and you are the owner of the document. When this error occurs, the Add-on displays the message below:
Unfortunately Google uses your "default" Google Account to assess if you are the owner of the document (not your "active" Google Account). So if you have multiple Google Accounts logged in at the same time and the default account is not the account you installed the Add-on with you could have an issue.
No worries though because we have a fix for you.
To resolve this error, please log out of your default Google Account then you will be able to use the Add-on with the account you installed it on.
You can find out who your default account is by clicking your circular user avatar in the top right hand side of google.
If you are using Google Chrome, another way round this is to have a separate Chrome profile (and Chrome window) for each of your google accounts. To try this method click link below and follow the steps for "Add a person or profile" google.com/chrome/answer/2364824