I was running an old version of Tab Slide. So I Deactivated it and Deleted it. Then I installed version 2.0.0.
So it works.
But on the Plugins page it says:
There is a new version of Tab Slide available. View version 2.0.0 details or update now.
And picking update now, it says it updated it but I still get that message.
Thank you in advance for your help.