Need to change instance family in aws ec2 savings plan or change the plan

Raise a support case with AWS as soon as possible to ask for a change to your plan. If you've only just purchased the plan there's a good chance AWS will agree though they're not obliged to.

Is there any error message while creating the EC2 instance - a1 in the AZ (availability zone)?

Here are some of the assumption: Is this a new AWS account? If yes, you may want to create small T2 micro EC2 first for 15-20 minutes then you should be able create the new a1 instance.

If this is existing AWS account and not limited capacity in the AZ, it could be due to service quotas or restriction on SCP in organization.

Service Quotas: https://docs.aws.amazon.com/general/latest/gr/aws_service_limits.html

SCP: https://docs.aws.amazon.com/organizations/latest/userguide/orgs_manage_policies_scps_examples_ec2.html