Can an instance reservation of Unix/Linux be applied to an instance running Ubuntu Pro AMI ?

If you are referring to Reserved Instance, it depends if it is purchased with Standard or Convertible type. Standard Reserved instances cannot be modified to be applied to other platforms. Convertible Reserved Instances can be modified to be applied to other platforms, though with some limitations. See:

• https://docs.aws.amazon.com/whitepapers/latest/cost-optimization-reservation-models/standard-vs.-convertible-offering-classes.html
• https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/reserved-instances-types.html
• https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ri-modifying.html

On the other end, Compute Savings Plans provide greater flexibility and help to reduce your costs. These plans automatically apply to EC2 instance usage regardless of instance family, size, AZ, Region, OS or tenancy, and also apply to Fargate or Lambda usage. For example, with Compute Savings Plans, you can change from Windows to Linux, shift a workload from EU (Ireland) to EU (London), or move a workload from EC2 to Fargate or Lambda at any time and automatically continue to pay the Savings Plans price.

• https://aws.amazon.com/savingsplans/
• https://aws.amazon.com/savingsplans/compute-pricing/

If you look closely at the pricing in Marketplace you will notice there is a Software charge and an EC2 charge that combine to the total. The RI should cover the EC2 portion of the charge, the Software portion will still bill and is not affected by the RI.

More detail in the breakdown on how it appears on the bill here.

Ok, so in order to match the platform for AMIs purchased via Marketplace, there is a specific field from AMI characteristics "PlatformDetails" that identifies the type of platform (usuallyt Linux/Unix). That platform is the one that can be used to purchase the correct Reserved Instance.

Here you can find more details: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ri-market-concepts-buying.html#ri-choosing-platform

"If you plan to purchase a Reserved Instance to apply to an On-Demand Instance that was launched from an AWS Marketplace AMI, first check the PlatformDetails field of the AMI. The PlatformDetails field indicates which Reserved Instance to purchase. The platform details of the AMI must match the platform of the Reserved Instance, otherwise the Reserved Instance will not be applied to the On-Demand Instance. For information about how to view the platform details of the AMI, see Understand AMI billing information. https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ami-billing-info.html"