I ran into this problem a few weeks back and again today when I tried to install Veeam Backup & Replication 5.0.2.
Product: Microsoft SQL Server Native Client — Error 1706. An installation package for the product Microsoft SQL Server Native Client cannot be found. Try the installation again using a valid copy of the installation package ‘sqlncli.msi’
This problem is caused by an existing installation of the SQL Native Client.
To fix this problem simply remove the SQL Native Client from the server and try to install Veeam again. Worked perfect for me.