Hi @mohitgh
when you initially sign up for QuickSight you have three options for user management
Use Active Directory
Use IAM federated identities and QuickSight-managed users
Use IAM federated identities only
I suspect the third option was picked. The most effective way to address this issue is to open a ticket with AWS customer support. They can then verify it via a screen share. To submit a ticket, please use this link. The support team can add the ability to add QS managed users if you open a support ticket in the AWS console. Give them your account number and tell them you want to add the ability to invite local QuickSight users via email to your account.
Refer this - Missing Invite User button