You have to define IP pools for your subnet.

(quantum) help subnet-create
usage: subnet-create [-h] [-f {shell,table}] [-c COLUMN] [--variable VARIABLE]
                     [--prefix PREFIX] [--request-format {json,xml}]
                     [--tenant-id TENANT_ID] [--name NAME]
                     [--ip-version {4,6}] [--gateway GATEWAY_IP]
                     [--no-gateway]
                     [--allocation-pool start=IP_ADDR,end=IP_ADDR]
                     [--host-route destination=CIDR,nexthop=IP_ADDR]
                     [--dns-nameserver DNS_NAMESERVER] [--disable-dhcp]
                     NETWORK CIDR

Create a subnet for a given tenant.

positional arguments:
  NETWORK               network id or name this subnet belongs to
  CIDR                  cidr of subnet to create

optional arguments:
  -h, --help            show this help message and exit
  --request-format {json,xml}
                        the xml or json request format
  --tenant-id TENANT_ID
                        the owner tenant ID
  --name NAME           name of this subnet
  --ip-version {4,6}    IP version with default 4
  --gateway GATEWAY_IP  gateway ip of this subnet
  --no-gateway          No distribution of gateway
  --allocation-pool start=IP_ADDR,end=IP_ADDR
                        Allocation pool IP addresses for this subnet (This
                        option can be repeated)
  --host-route destination=CIDR,nexthop=IP_ADDR
                        Additional route (This option can be repeated)
  --dns-nameserver DNS_NAMESERVER
                        DNS name server for this subnet (This option can be
                        repeated)
  --disable-dhcp        Disable DHCP for this subnet

output formatters:
  output formatter options

  -f {shell,table}, --format {shell,table}
                        the output format, defaults to table
  -c COLUMN, --column COLUMN
                        specify the column(s) to include, can be repeated

shell formatter:
  a format a UNIX shell can parse (variable="value")

  --variable VARIABLE   specify the variable(s) to include, can be repeated
  --prefix PREFIX       add a prefix to all variable names