See the compatibility notes in the SheepShaver for OSX download topic:
Quote:
- macOS 10.12 (Sierra) and later
There is an issue with macOS 10.12 Sierra (in fact with an additional new security feature in Sierra) that prevents a newly installed SheepShaver to run. The procedure to get it running is simply to copy the program (only the SheepShaver application itself!) to the desktop, remove the old program in the SheepShaver folder and move the new copy from the desktop to the SheepShaver folder.
(I will add a line there to make sure people will read all information.)