Excel does not provide a built-in way to return to the previously active worksheet. This VBA provides a method for doing this.

To use this solution, install the following code into the PERSONAL.xlsb workbook. There are three pieces of code that must be copied into three different places:

Create a new standard module with any name, and copy this code into it:
Option Explicit

' Provide the ability to create a Back button to return to the
' previously active sheet

' This code uses a class with state that tracks sheet changes for all
' open workbooks. If the VBA code is stopped (such as if an unhandled error
' occurs and the user hits End, or a developer clicks the Stop button
' in the VBA development window) then the tracker object is destroyed
' and will have to be started again. Note that this is done automatically
' on a failed attempt to go to the last sheet, but past information is lost.

Dim SheetTracker As New ClassSheetChangeTracker
   
' This instantiates the class used to track sheet navigation so
' must be called when this workbook is opened
Public Sub StartSheetTracker()
   Set SheetTracker.appevent = Application
End Sub

' Add a button to the Ribbon or the Quick Access Toolbar that references
' this sub to return to the previously active worksheet
Public Sub GoToLastSheet()
   
   On Error GoTo CannotDoIt
   
   ' An error will occur if the SheetTracker is not instantiated, or
   ' it cannot find the previous active sheet for the workbook
   SheetTracker.LastSheet(WorkbookName:=ActiveWorkbook.Name).Activate

   Exit Sub
   
CannotDoIt:

   If SheetTracker Is Nothing Then
   
      MsgBox "Sheet tracker not found, restarting"
      StartSheetTracker
   
   Else
      MsgBox "Unable to determine previous sheet for " & ActiveWorkbook.Name
   
   End If
   
End Sub
Create a new class called ClassSheetChangeTracker and paste the following code into it:
Option Explicit

Public WithEvents appevent As Application

Dim PreviousSheets As scripting.Dictionary


Private Sub Class_Initialize()

  Set PreviousSheets = CreateObject("Scripting.Dictionary")

End Sub

' This is an event callback function that will be called
' any time that a sheet in any open workbook is deactivated
Private Sub appevent_SheetDeactivate(ByVal Sh As Object)

   If PreviousSheets.Exists(key:=Sh.Parent.Name) Then
      PreviousSheets.Remove key:=Sh.Parent.Name
   End If
   
   PreviousSheets.Add key:=Sh.Parent.Name, Item:=Sh
   
   'MsgBox "Left sheet " & Sh.Name
   
End Sub

Property Get LastSheet(WorkbookName As String) As Worksheet

   Set LastSheet = PreviousSheets(key:=WorkbookName)

End Property
Add the following sub into the ThisWorksheet module. If you already have a sub called Workbook_Open in that module, add the line of code shown.
Option Explicit

Private Sub Workbook_Open()
   SheetTracking.StartSheetTracker
End Sub