Sub Datenimport()
Dim strInvoicePeriod As String
Dim File As String
Dim wkb As Excel.Workbook
Dim oBlatt As Worksheet
strInvoicePeriod = InputBox("Which Invoice Period shall be imported? (e.g. 02-2020")
Do Until strInvoicePeriod <> "" And Len(strInvoicePeriod) = 7
strInvoicePeriod = InputBox("Please insert the following data! Which Invoice Period shall be imported? (e.g. 02-2020")
Loop
File = Application.GetOpenFilename()
On Error GoTo ErrHandler
Application.ScreenUpdating = False
Set wkb = Workbooks.Open(File)
wkb.Sheets(1).Copy After:=ThisWorkbook.Sheets(ThisWorkbook.Sheets.Count)
ThisWorkbook.Sheets(ThisWorkbook.Sheets.Count).Name = strInvoicePeriod
wkb.Close False
Application.ScreenUpdating = True