AADDS is not accepting prefix more than 15 charcters.

gadmin285 Unfortunately this is a NetBIOS limit which has been around for a long time, and probably will not change. 

The 15 character limit is documented here https://docs.microsoft.com/en-us/azure/active-directory-domain-services/tutorial-create-instance "Domain prefix restrictions: You can't create a managed domain with a prefix longer than 15 characters. The prefix of your specified domain name (such as aaddscontoso in the aaddscontoso.com domain name) must contain 15 or fewer characters."