Check if current (active) cell is empty

IsEmpty doesn't check what you think it does, but instead merely verifies if a given variable has been initialized.

Instead, check for an empty string.

Sub Lateralus()

Range("C1").Select
Selection.End(xlDown).Select
X = ActiveCell.Offset(0, -1).Select

' Test if the value in neighboring cell is blank/empty
If x = "" Then MsgBox "Cell is empty"

End Sub