Hi @Rajadurai V ,
I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others ", I'll repost your solution in case you'd like to "Accept " the answer.
Issue:
When trying to add a Custom Attribute in the User Signup (userName and phoneNumber) in Azure AD B2C, you received the following error message:
{"status":"400","errorCode":"AADB2C90278","message":"Unable to validate the information provided."}
Solution:
You made sure that the userName and phoneNumber custom attributes were added in all of the steps and were able to resolve the issue.
Please remember to "Accept Answer" if any answer/reply helped, so that others in the community facing similar issues can easily find the solution.