Not all EC2 instance types are available in all availability zones in a region. If you want to check in which availability zones of a region a particular instance type is available, you can use the following command:
EXPORT TYPE=<instance type> # e.g. EXPORT TYPE=t3a.medium
aws ec2 describe-reserved-instances-offerings --filters 'Name=scope,Values=Availability Zone' --no-include-marketplace --instance-type $TYPE | jq -r '.ReservedInstancesOfferings[].AvailabilityZone' | sort | uniq