Please ensure that you have selected Unverified backup set and teh option should become available.
Please refer https://library.netapp.com/ecmdocs/ECMLP2353675/html/GUID-6C829C9B-A6CF-4BFF-9142-E6ACBE036522.html
https://library.netapp.com/ecmdocs/ECMLP2353675/html/GUID-672CD0FF-2A5D-4F6F-8337-8DA7B9674D6D.html