I also met the same problem and I finally passed it following these steps according to the accepted answer of Post1 and Post2

My steps: 

1. installed a package with id   "04t0P000000N9rs" , I used to use " 04t41000002Q3o0".  

(ref: Post1) 

2. change the field type (ref: Post1) 

* it says "

" but I can only find " Visible" and "Read-only" . I checked these two. 

**you can take a reference of Post2 to make it clear of the reason of changing the field type first  

the exercise again 

It works . You can take a try.