How do you unprotect variables so that they can be used in computations. I used the Unprotect[ ] command and I get variable results in the application; see the attached nb.
In your notebook your variable eta (I am writing it out in latin letters here but you are using the greet eta) does not need to be unprotected. It was never protected.
However, you will note that what appears to be the symbol eta in your notebook is actually not symbol: it is a hyperlink to the documentation for eta. This is why you are getting your various error messages. Enter an eta either by typing
or by typing any of the keyboard combinations described here:
I did as you suggested and the Greek symbol appeared. I used it to copy and paste where the symbol was needed and it worked great.