Olá Jefferson TS
Altere esta parte do código e veja se resolve:
Dim PLAN As String
PLAN = UserForm1.ComboBox1.Text
'fonte https://docs.microsoft.com/pt-br/office/vba/api/excel.range.find
dia = CDate(UserForm1.TextBox8.Text) 'formatar para data
Worksheets(PLAN).Activate
'linha = Cells.Find(dia).Row
'
'ActiveSheet.Cells(linha, 7) = UserForm1.TextBox1.Text
'ActiveSheet.Cells(linha, 8) = UserForm1.TextBox2.Text
'ActiveSheet.Cells(linha, 9) = UserForm1.TextBox3.Text
'ActiveSheet.Cells(linha, 10) = UserForm1.TextBox4.Text
'ActiveSheet.Cells(linha, 11) = UserForm1.TextBox5.Text
'ActiveSheet.Cells(linha, 12) = UserForm1.TextBox6.Text
Dim rgn As Range
With Worksheets(PLAN).Range("G:G")
Set rgn = .Find(dia, LookIn:=xlValues)
If Not rgn Is Nothing Then
linha = rgn.Row
Do
ActiveSheet.Cells(linha, 7) = UserForm1.TextBox1.Text
ActiveSheet.Cells(linha, 8) = UserForm1.TextBox2.Text
ActiveSheet.Cells(linha, 9) = UserForm1.TextBox3.Text
ActiveSheet.Cells(linha, 10) = UserForm1.TextBox4.Text
ActiveSheet.Cells(linha, 11) = UserForm1.TextBox5.Text
ActiveSheet.Cells(linha, 12) = UserForm1.TextBox6.Text
Set rgn = .FindNext(rgn)
Loop While Not rgn Is Nothing
End If
End With
If CheckBox1.Value = True Then
ActiveSheet.Cells(linha, 6) = "X"
End If
Veja se é por aí......