The error you're encountering indicates that your account has reached a limit for purchasing Reserved Instances, but as you've correctly identified, the standard self-service quotas don't explain the block. Based on the error code 'ReservedInstancesLimitExceeded,' your current quota does not allow you to purchase the required number of Reserved Instances, even though your visible quotas show available capacity.

While I cannot see the specific internal restriction affecting your account, there are a few possibilities for what might be causing this:

First, there may be account-level purchasing restrictions that aren't exposed in the Service Quotas console. These can sometimes apply to newer accounts, accounts with limited payment history, or when purchasing No Upfront Reserved Instances, which carry higher financial risk for AWS since there's no upfront payment.

Second, GPU instance families may have additional purchasing controls that differ from general-purpose instances. These specialized instance types can have separate reservation limits that aren't always visible in the standard quota displays.

Third, regional availability can play a role. Certain instance families may have restricted Reserved Instance availability in specific regions, particularly for newer GPU types or in regions with limited capacity.

To resolve this issue, you'll need to work directly with AWS Support through your existing case. Since you have a Basic support plan, response times may be longer, but the support team will be able to see account-level restrictions that aren't visible to you. When you add correspondence to your case in the AWS Support Center, be sure to include all the details you've provided here—the specific instance type, term length, payment option, region, and the quota checks you've already performed. This will help them investigate the root cause more quickly.

If faster response times are critical for your business needs, you might also consider reviewing your AWS Support plan options, as higher-tier plans provide more rapid responses for these types of issues.
Sources
Error codes for the Amazon EC2 API - Amazon Elastic Compute Cloud
PurchaseReservedInstancesOffering - Amazon Elastic Compute Cloud

Hi there.

I apologize for trouble you're experiencing with the error message when purchasing a Reserved Instance. I've shared your sentiments internally with our Support team.

Continue monitoring your case as correspondence will be shared once the review is complete.

— Roman Z.