SAML for the Customer Portal is dependent on SAML being correctly configured for the Salesforce organization that contains the portal
First setup Salesforce SAML in Okta with your main Salesforce instance and note the Issuer ID
Use that same Issuer ID in the configuration for the Customer Portal.
If you need to embed deeplinks into the portal, you can use this as the base URL:
Sign in to the Okta Admin app to have this variable generated for you