Like @HansS said. The best thing to learn about KNIME is to start doing something. A good starting point could be to take Kaggle competition and try to do everything in KNIME (from scratch) - this is how I started using it more. Or try to bring one of your daily tasks from Excel to KNIME. You will encounter all kinds of challenges but with the powerful resources of KNIME, you will overcome them and learn a lot.
Having said that. There is a free Udemy video course when you like this kind of learning:
https://www.udemy.com/course/knime-bootcamp/learn/
Then of course there is the KNIME press with books for starters and more advanced stuff. And also guides if you are familiar with Excel or SAS and want to start using KNIME.
Then I might humbly point you to my own repository on the hub where I cover a wide range of topics. Some inspired by questions from the forum some from my more general use of KNIME. All examples contain sample data and can be completely run. And mostly I have enclosed descriptions or links to the relevant discussions in the forum so although not a complete course they might give you more insights about what you can do with KNIME.
And then on the hub there is so much more to find. I will just add my collection of interesting collections and some blog posts. But this is just a small insight into what is possible.
KNIME User Training Sample Workflows
https://hub.knime.com/knime/spaces/Education/latest/01%20KNIME%20User%20Training/
extract_from_knime_hub.docx (12.3 KB)