Ah, your variable is not the MultpackageDetailsRecord, but one of its attributes. I was already wondering...
Ok, that's a pretty complex screen. It may be that the Platform somehow gets confused as to what inputs need validation triggered by what link or button. As I can't debug it, as you rightly pointed out already, I can't work out why the client validation doesn't work*.
However, if you need server validation, then it's pretty simple: if the variable is 0, then there's nothing selected, and you can take the appropriate action (e.g. show a message).
*What you could do is copy and strip the screen, and see if it still doesn't work in a very simple case, that is a screen with a single button with Client/Server validation.
Ok, that's a pretty complex screen. It may be that the Platform somehow gets confused as to what inputs need validation triggered by what link or button. As I can't debug it, as you rightly pointed out already, I can't work out why the client validation doesn't work*.
However, if you need server validation, then it's pretty simple: if the variable is 0, then there's nothing selected, and you can take the appropriate action (e.g. show a message).
*What you could do is copy and strip the screen, and see if it still doesn't work in a very simple case, that is a screen with a single button with Client/Server validation.