I have the same issue you can check :

https://www.keycloak.org/server/features#:~:text=Multi%2Dsite%20support-,Preview%20features,-Preview%20features%20are

I just restart my keycloak environnement and add KC_FEATURES=token-exchange to enabled the feature.

(I use chart helm to deploy)

containers:
 - name: my_container_name    
   image: "quay.io/keycloak/keycloak:{{ .Values.KC_version }}"
      command:
        - /opt/keycloak/bin/kc.sh
        - start
      env:
        ....
        - name: KC_FEATURES
          value: token-exchange