These videos are free and are good for start:
This is a comprehensive list of nifi tutorials: https://github.com/jfrazee/awesome-nifi
Nifi docs is also a good source: https://nifi.apache.org/docs.html
If you are looking for beginners' course: https://www.udemy.com/apache-nifi/
You can check my YouTube playlist. I have created it, especially for beginners.
After finishing my YouTube tutorial, if you wish to dive deep into the advanced topic, you can opt my Udemy course.
You can learn the same course in Skillshare for FREE using the below referral link.