Cannot create key pair for ec2 instance.

Is the permission to create key pairs attached to your IAM user, etc.?
If ec2:CreateKeyPair is not attached to the IAM policy, an error will occur.
https://docs.aws.amazon.com/service-authorization/latest/reference/list_amazonec2.html

Log in to the AWS Management Console and navigate to the IAM service. Select the relevant user for whom you want to add the policy. Click on the "Permissions" tab. Click on the "Add permission" button. On the next page, select the option "Attach existing policies directly". In the search bar, type "CreateKeyPair" and select the policy from the list of results. Review the policy details and click on the "Add permissions" button to attach the policy to the user. Reference: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-key-pairs.html

Hi. What you have attached is ECSFullAcess(not EC2FullAccess) and it seemes no ec2:CreateKeyPair action included on it.

https://docs.aws.amazon.com/AmazonECS/latest/userguide/security-iam-awsmanpol.html