Compatibility between Rocky and hpc* instance types

The incompatibility you're experiencing between Rocky Linux 9 and the Hpc7a instance types is not due to an architectural mismatch or Rocky Linux's suitability for HPC workloads. Instead, it's likely related to the availability of Hpc7a instances in your chosen region.

Hpc7a instances are not available in all AWS regions. According to the information provided, Hpc7a instances are currently available in US East (Ohio), Europe (Ireland), and AWS GovCloud (US-West) regions. The screenshot you've shared indicates that you're trying to launch the instance in the eu-west-1 region, which is not listed as a supported region for Hpc7a instances.

To resolve this issue, you should try launching your Rocky Linux 9 instance with Hpc7a in the Europe (Ireland) region, which is eu-west-1. If you specifically need to use the eu-west-1 region, you may need to choose a different instance type that is available in that region.

It's important to note that the compatibility issue is not related to Rocky Linux itself or its suitability for HPC workloads. Rocky Linux, being an enterprise-grade Linux distribution, is generally compatible with various instance types, including those optimized for HPC workloads. The issue you're encountering is more likely due to the regional availability of the specific Hpc7a instance type.
Sources
Announcing Amazon EC2 Hpc7a instances
Deep-dive into Hpc7a, the newest AMD-powered member of the HPC instance family | AWS HPC Blog