If you get the message “Project is not available” this may mean that:
The project owner has removed you from the project. You can to see if you have access to the project through the web app. If the project is not displayed in the list of your projects then you don't have access to the project you are looking for.
You have checked and found out that you have access to the project. In this case you need to log out in the Plant plugin, log in again and try to open the project.