I have the same problem I deleted the default VPC and everything linked to it while doing some clean up (when I was getting charged for something), Sure enough I ignored the warnings. Upon looking in to the documents it turns out that it is just a pre defined VPC for quick usage u can create one and use it like default VPC by looking at the specs here

https://docs.aws.amazon.com/vpc/latest/userguide/default-vpc-components.html