Hello Madhura Sane
I hope answer provided by @TP is helpful.
Additionally, as I understand you have Global admin role assigned for your account for your tenant and you are unable to assign role assignment to your subscription.
When you try to add IAM assignment the option is disabled.
If you have a Global admin role assigned, this means you are complete admin in Entra ID within Azure. Entra ID is a directory within Azure. With Global admin permissions you can perform anything within the directory like, created users, deleting users, password resets, registering applications etc.
If you want to perform anything with respect to subscription or resources in Azure, then you need to have an IAM role assigned. Example roles, Owner, contributor, User access administrator etc.
You can go through below article to get more information on in-built role in Azure IAM,
https://learn.microsoft.com/en-us/azure/role-based-access-control/built-in-roles
There are multiple roles within Azure with permission defined that will allow you to perform different actions on multiple resources.
As explained earlier, Azure roles are completely different from Entra ID roles. Global admin is a role which is part of Entra ID.
Hope this information helps. Let us know if you have any additional queries. Happy to assist you further.