Have a VPC using a 10.1.0.0/16

Hi
Something to check.
On the VPC Console, check your Subnets tab to make sure they are as expected. My guess is that 10.0.0.0/16 exists in a VPC somewhere and has been sliced into 3 subnets of /20 already and probably conflict with your attempts to allocate those subnets.
If using the AWS CLI, try aws ec2 describe-subnets -text and check the output for the same.

Or follow this guide to create a new VPC with new subnets and you should get your outcome. (Try it in a new region to test it out if you want).
http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/getting-started-ipv4.html
HTH