If you have already uploaded the app to Play store you can generate Hash Key as follows:
Go to Release Management here
Select Release Management -> App Signing
You can see SHA1 key in hex format App signing certificate.
Copy the SHA1 in hex format and convert it in to base64 format, you can use this link do that without the SHA1: part of the hex.
Go to Facebook developer console and add the key(after convert to base 64) in the settings —> basic –> key hashes.