@John_khan hi!
From the information you provided, the IP address for loopback10 is 172.16.10.1/16 and the IP address for loopback11 is 172.16.11.1/16.
Both IP addresses belong to network 172.16.0.0/16. So, the error message appears when you configure the IP address 172.16.11.1/16 to the interface loopback11.
You can modify the IP address for loopback11 to 172.17.11.1/16, or you can modify the mask length for interface loopback10 and loopback11 to 24, rather than 16.
Both ways can solve this problem.