LoyaltyLion requires access to your REST API.
It's quite common to control access to your store with HTTP Basic Auth during development. If you have done this, you'll need to remove this extra authentication on your API routes.
Example .htaccess configuration:
# set an environment variable "noauth" if the request starts with "/api/rest"
# Magento 1.x
SetEnvIf Request_URI ^/api/rest noauth=1
# Magento 2
SetEnvIf Request_URI ^/rest noauth=1
# the auth block
AuthName "Please login."
# Here is where we allow/deny
Deny from all
Allow from env=noauth