Get session token to SSO to Sisense application

Can we fetch session token for an IDP authenticated user as it doesn’t have any password or credentials to login

No, you can only get a sessionToken from the /authn endpoint. Users logging in through an IdP will have their session cookie set for them by the Okta domain once Okta is able to confirm that user has logged into their IdP.

How can we use session cookie in SSO user into Sisense

I don’t understand, what is Sisense? Is this an OIN app? A Custom SAML app?

Sisense is an app where we login using saml url appended with dynamic oauth session token which we fetch with /authn api

Eg:https://vartopia.okta.com/app/sisense/exk5zbp437x1Q39Hr2p7/sso/saml?sessionToken=20111ZaKUu3lnho60rjbwtbLL26FNcI4x6-uDAg-xgD_TRdo6VUT5OR&RelayState=https://stage.vartopia.com/VartopiaV2/AdminHome

If the user already has a session with Okta in their browser, you can omit the sessionToken portion of the URL to log the user into the SAML application

You mean in case of IDP authenticated user should be logged in browser?

Yes, IDP users have to be logged in via the browser, since they do not have a password with Okta you will not be able to log them in via /authn to get a sessionToken. Instead the user should first be logged into Okta via their IdP and THEN you can make a request to SSO the user into the SAML application within the Okta Org