To create a subnet inside a VPC using AWS CLI you can use the following command:-
Pre-requisite:-
- AWS CLI configured
 
- VPC ID
 
The command used is as follows:-
C:\Users\priyj_kumar>aws ec2 create-subnet --vpc-id vpc-0aca683c37231c12f --cidr-block 10.0.1.0/24
The output you will get after executing the command will be as follows:- 
{
    "Subnet": {
        "AvailabilityZone": "us-east-1e",
        "AvailableIpAddressCount": 251,
        "CidrBlock": "10.0.1.0/24",
        "DefaultForAz": false,
        "MapPublicIpOnLaunch": false,
        "State": "pending",
        "SubnetId": "subnet-08f4ee13b627d22bb",
        "VpcId": "vpc-0aca683c37231c12f",
        "AssignIpv6AddressOnCreation": false,
        "Ipv6CidrBlockAssociationSet": []
    }
}
This way you can create a subnet inside a VPC. Hope this helps.