Hello @gtlane ...
What happens when you click on the "browse" link to search for your course sections? Do you see any of the sections that you have listed in your second screen shot? What if you try and type the name of the section into the text box next to the word "browse", does that work? Also, have you spoken with your school's local Canvas administrator or someone in your school's Online Learning/eLearning department to see if they would be able to help you if you aren't able to complete this yourself?
Hope to hear back from you soon, Gloria. Be well, and stay safe!
I was able to add and remove students from sections in a course. The screencast I recorded is a rough overview of the process but I think it can be helpful.
Reminder, I did this in Beta so I am not aware of the effects it will have in production.
If someone has an answer that'd be great!