backup user permission in vCenter

Hi

I use HP MSL 4048 with HP Data Protector to backup my data in our company

I want to grant permission to backup user to connect to vCenter and backup VMs. already,in our environment,this user has administrator privilege and everything is good but i want to give it minimum access to VC. 

There is a predefined role in VC titled as "backup consolidated user" that when i add my backup user in it , it can not login to VC and give me "login failed" error. 

thanks for replies