Hello Muhamad Dimas Hadi Akrom
Welcome to Microsoft Q&A Platform, thanks for posting your query here.
If you are unable to see the Southeast Asia region in the list of available regions when creating a VM, it is possible that the region is currently unavailable or has limited capacity. And other reason could be the VM size you are trying to deploy may not be available in the region.
Please check if the VM size is available in the region from the below link:
If the VM size is showing as available for the region, then it could be capacity issue. You can try deploying in another region. You can check the Azure status page to see if there are any known issues or outages in the region.
If the VM size is not available for the region, request you select VM size that is available in the region and try deployment.
Hope this helps.