To deploy FortiGate on the marketplace, you must log into the AWS portal as an AWS user. Your organization's administrator may have granted permissions via certain IAM roles. AWS security best practices explains when and in what use cases you need IAM roles. How you manage IAM users and roles is up to your organization.
When deploying FortiGate on Marketplace web or EC2 console, your AWS account must have appropriate permissions, including being able to subscribe to AWS resources through the marketplace, access EC2 resources, browse AWS resource groups, and so on.