Insufficient-capacity when trying to add a Reader to ap-southeast-2a Aurora Postgres of Size db.r8g.large

The error you're encountering, "Insufficient-capacity," indicates that AWS currently doesn't have enough available capacity to provision the db.r8g.large instance type in the ap-southeast-2a Availability Zone for your Aurora PostgreSQL Reader.

This situation can occur due to high demand for specific instance types in a particular Availability Zone. It's not uncommon, especially for newer or specialized instance types like the R8g family.

Here are some troubleshooting suggestions:

1. Try a different Availability Zone: Attempt to create the Reader in a different AZ within the ap-southeast-2 region, such as ap-southeast-2b or ap-southeast-2c.

2. Use a different instance type: Consider using an equivalent instance type from another family, such as db.r6g.large or db.r5.large, which might have more available capacity.

3. Wait and retry: Sometimes, capacity issues resolve themselves as resources become available. Try again after a few hours or the next day.

4. Request a capacity increase: If this is a recurring issue, you can submit a request to AWS Support for additional capacity in your preferred AZ.

5. Use Aurora Multi-AZ: If you haven't already, consider setting up your Aurora cluster as Multi-AZ, which automatically distributes instances across different AZs for better availability.

6. Explore Aurora Serverless v2: If your workload allows, Aurora Serverless v2 can automatically scale capacity and might be less susceptible to capacity constraints.

Remember, while you're troubleshooting this issue, your existing Writer instance should continue to function normally. The inability to add a Reader doesn't affect the operation of your current Writer.

If the problem persists after trying these suggestions, it would be advisable to contact AWS Support for further assistance, especially if you have critical business needs requiring the additional Reader instance.
Sources
Aurora PostgreSQL Writer and Reader Region & AZ both shows as being at ap-southeast-2b. Is this an issue that has to be corrected? | AWS re:Post
Can I fail over a writer instance to a new and scaled up reader instance in an Single-AZ Aurora cluster? | AWS re:Post