It is as simple as that !
“Learning new words” button became inactive/disabled after my premium subscription ended.
This happened on one course: 400 words of TOEFL
Please Help
Thanks
Could it be that you have completed planting the course?
Does the course content screen show everything as planted?
Judging from your word count in your screenshot, I have a strong feeling ignore words is broken. 
I’ll notify the Android team to have a look into this.