We can see the following error message while scaling the SQL Managed instance.
Error message:
Cannot perform creation/scaling of the managed instance as there are not enough available IP addresses in the subnet for performing the operation. For more details visit ‘https://aka.ms/determine-subnet-size‘.
Solution:
We need to move the SQL MI to another subnet which is having the sufficient IP address using the terraform.
Currently, there is no way to complete the scaling operation in case there aren’t enough IP addresses available. It’s also not possible to change the subnet address range if any resource exists in the subnet.
Careful planning of subnet size for your managed instance deployments is recommended.
We suggest that the new subnet is created with more IP addresses allocated so future update operations will avoid similar situations.
For more details visit:
Determine required subnet size & range for Azure SQL Managed Instance .
Azure SQL Database 