@zielinska_km check if you have installed the extension and maybe provide a sampel workflow that shows what you are doing. Also make sure you have the latest KNIME version.
We can only guess. If you share workflow example I’m sure someone will take a look at it and help you. Simply put some data into Table Creator node and follow it with node that gives you above mentioned error. This topic might help you:
Sorry for the trouble here, I will file a ticket (EDIT: filed, AP-19544) and ask our developers to look into this strange behavior with the Expressions nodes. In the meantime, is it possible that a fresh installation of KNIME AP will resolve the issue? I have a suspicion that this maybe due to a corrupted update from a previous version of KNIME - but this is only a suspicion and I may be wrong.
Can I also ask each of you to confirm which version of KNIME AP and operating system you are using?