Try this
BTW your description said "decreasing" but your macro said "xlAscending"
Option Explicit
Sub SortSheetsColR()
Dim ws As Worksheet
Dim r As Range, r1 As Range
Application.ScreenUpdating = False
For Each ws In Sheets
With ws
Set r = .Cells(1, 1).CurrentRegion
Set r1 = r.Cells(2, 1).Resize(r.Rows.Count - 1, r.Columns.Count)
With .Sort
.SortFields.Clear
.SortFields.Add Key:=r1.Columns(18), Order:=xlAscending
.SetRange r
.Header = xlYes
.Apply
End With
End With
Next ws
Application.ScreenUpdating = True
End Sub