Hi @Abirami_SD,
In CRXDE, go to /content/experience-fragments and check the "cq:allowedTemplates" property

Also to add an experience fragment inside an experience fragment, edit the template and add the "Experience fragments" in the Layout container policy. This is similar to adding the allowed components in the container policy.

Also, it is recommended to try all these in touch UI as they are the latest functionalities by adobe.
Hope this helps.
Thanks,
Kiran Vedantam