You need to put the worksheet identifier in your range statements as shown below ...
Option Explicit
Dim ws As Worksheet, a As Range
Sub forEachWs()
For Each ws In ActiveWorkbook.Worksheets
Call resizingColumns
Next
End Sub
Sub resizingColumns()
ws.Range("A:A").ColumnWidth = 20.14
ws.Range("B:B").ColumnWidth = 9.71
ws.Range("C:C").ColumnWidth = 35.86
ws.Range("D:D").ColumnWidth = 30.57
ws.Range("E:E").ColumnWidth = 23.57
ws.Range("F:F").ColumnWidth = 21.43
ws.Range("G:G").ColumnWidth = 18.43
ws.Range("H:H").ColumnWidth = 23.86
ws.Range("i:I").ColumnWidth = 27.43
ws.Range("J:J").ColumnWidth = 36.71
ws.Range("K:K").ColumnWidth = 30.29
ws.Range("L:L").ColumnWidth = 31.14
ws.Range("M:M").ColumnWidth = 31
ws.Range("N:N").ColumnWidth = 41.14
ws.Range("O:O").ColumnWidth = 33.86
End Sub