instance availability

The issue you're experiencing is likely related to account-specific capacity or availability constraints in the specific Availability Zone for your problematic account. Even though the instance type (r5.xlarge) is generally available in that zone, as evidenced by its availability in your other accounts, AWS may have limited capacity for that instance type in that particular zone for your specific account.

This situation can occur due to various reasons:

1. AWS might be managing capacity differently across accounts to ensure fair distribution of resources.

2. There could be temporary capacity constraints in that Availability Zone for your account.

3. Your account might have a lower priority for new instance types in that zone due to usage patterns or account history.

To resolve this issue, you can try the following:

1. Wait and try again later. Capacity fluctuates, and the instance type may become available in that zone for your account.

2. Request a limit increase for that instance type in the specific region. Even if you're not hitting a limit, this can sometimes trigger a review of your account's capacity allocation.

3. Try launching the instance in a different Availability Zone within the same region where the instance type is available.

4. If possible, use a different instance type with similar specifications that is available in your desired Availability Zone.

5. Contact AWS Support to inquire about the specific capacity constraints for your account in that Availability Zone and see if they can assist in resolving the issue.

Remember that instance type availability can vary not just by region and zone, but also by individual AWS account, which is why you're seeing different results across your accounts.
Sources
Amazon EC2 instance types for SAP on AWS - General SAP Guides
Troubleshoot changing the instance type - Amazon Elastic Compute Cloud
Troubleshoot a missing or stuck Amazon RDS DB instance class | AWS re:Post
Troubleshoot Amazon EC2 instance launch issues - Amazon Elastic Compute Cloud

Hello,

adding to the above, please note that the mapping of Availability Zone names to the underlying physical data centers is not consistent across AWS accounts. This means that for the same Region (e.g., us-east-1), the Availability Zone "us-east-1a" may refer to a different physical location for different AWS accounts. In order to avoid confusion, you instead of relying on AZ names, you can use Availability Zone IDs to ensure consistency across accounts.