Hi Samirit Saha,
Thanks for using Microsoft Q&A!!
You are getting this error as Completion operation is not supported by gpt-35-turbo(0613) and gpt-35-turbo-16k(0613) models. These models only support Chat Completions API. Only older turn model - GPT-3.5 Turbo (0301) supports both Chat and completions API. Please refer to the GPT-3.5 models for details.
Additionally, Models page of Azure AI Studio displays the features supported by the Model as shown in the below screenshot -
Please let me know if you have any other questions.
Please do not forget to "Accept the answer" wherever the information provided helps you to help others in the community.