Hello sourabhpanse-6566, Sorry this has been stuck for months. Here are a few things to try.
Curious if you are able to reproduce the registration error from a different machine/client and browser?
Confirm you have the appropriate level of access to manually register the Microsoft.Tokens RP.
Navigate to Resource manager explorer blade and check if you are able to drill into the RP elements as shown below.
If your Azure subscription has multiple administrators/owners, ask them to follow the same steps to see if this issue is widespread. If issue persists, don't hesitate to reply and we can assist with engaging the Azure technical support channel for further investigation.