Hi Pamitha Herath,
Appreciate your effort is sharing the solution/suggestion for the question so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others ", I'll repost your solution which is product limitation in case you'd like to "Accept " the answer.
Solution:
Bicep isn't supported for enabling autoscale on an existing database or container and only supported Azure portal , powershell and Azure CLI according to below
If you have any other questions or are still running into more issues, please let me know. Thank you again for your time and patience throughout this issue.
Please remember to "Accept Answer" if any answer/reply helped, so that others in the community facing similar issues can easily find the solution.