this can be actual more issue than just one. What you try to restore and how you did the settings for File -> “Specify Netbackup Machines and Policy Type” ?
It can also be a network issue like DNS or stuff like this what can generate this Error.
So the first you have to check is if the current settings are there and that the network is working for the involved server.
From the Master server -> open the admin console --> host properties -> master servers
Go to client attrubites , select your client ( or add it ) under general make sure browse and restore ability has been selected to allow.