Azure Account "Add Role Assignment" Disabled

As I mentioned in the comments, to assign roles to any resource in Azure Portal, you should have either Owner or User Access Administrator roles on your Azure subscription.

• Microsoft.Authorization/roleAssignments/write permissions are included in these roles that allows to assign roles.
• Global Administrator has the rights to assign these roles.
• As you have only Contributor role, you cannot assign roles, but you can create and manage resources.
• You must contact the person with Global Administrator/Privileged Role Administrator role to assign those permissions for you.

Please check the below from MsDoc:

After acquiring any of those 2 roles, Add role assignment option will be enabled.

You can check the below references for more details:

Assign Azure roles using the Azure portal - Azure RBAC | Microsoft Docs

Assign Azure AD roles to users - Azure Active Directory - Microsoft Entra | Microsoft Docs