Simplesamlphp gives error while trying to access the login url

Question

I am trying to use SimpleSAMLphp (https://simplesamlphp.org/) for setting up SSO on our website. This will be IdP initiated login. I have configured it on our website subdomain idp.domain.com. I am able to access the fronend, and admin. All the requirment mentioned are being fullfilled as per checks in admin. However when trying to access the login url it gives the 403 forbidden error.

The url is https://idp.DOMAIN.com/saml-Idp/simplesamlphp/saml2/idp/SSOService.php?spentityid=SERVICE_PROVIDER_ENTITY_ID

Also it doesnt matter, any SP Entity id passed gived the same error. What could be the issue?