Thunderbird - "No writable calendars are configured..."

Hi all; I have Thunderbird 52.9.1 installed on Fedora 28 (KDE Spin), every invite I get for a meeting shows this : "No writable calendars are configured for invitations, please check the calendar properties" I have the "Provider for google calendar" add on installed, I created my calendar by add calendar -> on the network -> google calendar Thoughts / ideas? Thanks in advance for your help...