Keycloak 16 Enabling offline access in client scope

Question

In my project, we are using keycloak (16.1.1) as IDP. We are facing an issue when the server restarted all the sessions will be removed. to resolve this I'm trying to generate tokens with passing scope as scope = offline_access.

1.I assigned offline access to the client scopes as below

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2.I tried to generate a client token with passing scope as scope = offline_access and I got an error below { "error": "not_allowed", "error_description": "Offline tokens not allowed for the user or client" }

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3.I checked my error logs from the server and I got the error below KC-SERVICES0060: Role 'offline_access' not available in the realm

I'm looking for advice, on how to add the Offline access in client scope for keycloak 16.1.1.