strData() cannot be used without a Redim as mentioned in post#2.
However, you don't need strData() to add those captions into Listbox.
Code:
Option Explicit

Private Sub Command1_Click()
    Dim IntCounter     As Integer
                
    For IntCounter = 0 To 4
      If txtDana(IntCounter).Text <> "" Then
        List1.AddItem lblZwierzaki(IntCounter).Caption
      End If                               
    Next
      
End Sub