hello @Anonymous - Yes, ACM supports creating cost alerts for container instances.
I was able to use the In-Portal workflow to create cost alert for a container instance- ref screenshot below.
Can you confirm you have at least 1 container instance deployed to the the management scope/subscirption refrenced in your budget?
You may also want to check that you have appropriate permissions needed to manage cost on the desired management scope.
Let me know if you need further assistance. happy to help.
Cheers