Well it seems like you have migrated to the Ninja forms 3 and in ninja forms 3 seems like they have removed the filter that we were using to show the make Instructor button .
So I roll back from ninja forms settings and make instructor button began to show but this removed your existing Ninja forms .
so create a new one and test this without migrating to ninja forms 3 .
I have tested this on my test site .
Also adding this to issue log .
We will be making our make instructor feature compatible with the Ninja forms 3.
Thanks for reporting this to us .
track :
https://trello.com/c/waFkg18J