Hi,
In order to configure a DHCP server pool, are you following below commands:
Have you configured DHCP address pool?
set access address-assignment pool pool-v4-def family inet network 20.1.1.0/24
set access address-assignment pool pool-v4-def family inet range r low 20.1.1.101
set access address-assignment pool pool-v4-def family inet range r high 20.1.1.200
Please follow below link:
https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/dhcp-server-cli.html
-Ajo Thomas