Rows 52081 and 52083 in the Big Sample.xlsx have a #NAME error in column B so that was the Type Mismatch error
Maybe this - only took 3-4 seconds on the large file
Option Explicit
Sub Change_Text()
Dim rCell As Range
Application.ScreenUpdating = False
For Each rCell In Worksheets("Sheet1").UsedRange.Cells
If VarType(rCell.Value) <> vbString Then GoTo GetNextCell
If LCase(Trim(rCell.Value)) <> "english section details" Then GoTo GetNextCell
If LCase(Trim(rCell.Offset(1, 0).Value)) <> "term" Then GoTo GetNextCell
rCell.Offset(1, 0) = "English Terms"
'rCell.Offset(1, 0).Interior.Color = vbRed <<<< just for testing
GetNextCell:
Next
Application.ScreenUpdating = True
End Sub