This would really be better if you were using Content Controls. I think you should either:
1. Change to content controls or
2. Change your checkboxes to option buttons
The only benefit of using legacy fields over Content Controls is the availability of legacy Option buttons and the ability to group them so only one can be turned on at a time. You are trying to replicate that built-in functionality with code because you chose a checkbox instead of an option button.
But doing it with the legacy checkbox field, for that macro you just need a single line
Code:
Private Sub LPja_Click()
ActiveDocument.Bookmarks("TabelLP").Range.Font.Hidden = Not LPja.Value
End Sub