Maintenance Cleanup Job Task Fails but Backup Jop is successful
Much as the error message states, it can't find the file N'\\SQLSvr..." 

If you know the file does exist, then that usually points to a permission problem with either the account SQL Agent is executing as or if you've created it, a credentials proxy account. Either way, it can't access the network share or it does not have permission to perform the delete operation. 

I'd first verify the account can access the share, then look at folder permissions and ensure it has delete privilege.