Hi @Shubham Yadav,
Thanks for reaching Microsoft.
I tried this API, but I was not able to replicate this issue. I was able to delete the event attachments successfully with the below API and received "HTTP/1.1 204 No Content" response.
DELETE /me/events/{eventId}/attachments/{attachmentId}
As this seems to be a tenant specific issue, I would recommend you to raise a support case with Microsoft Graph, a Support Engineer will be able to assist you better. You can raise support ticket from New support request - Microsoft Entra admin center or https://admin.microsoft.com/#/support/requests.
Hope this helps.
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