Method `Range of object_'Worksheet' failed - Trying to write variable to cell

Hi All - Hoping someone can assist.

I am trying to update a spreadsheet I have used for carrying out a wall thickness calculation on piping, and thought it would be a great idea to try and learn VBA at the same time. I have managed to put together some code that generally does what I need (basically finding values in a table of data and interpolating to get intermediate figures) but I am getting an error trying to write the result to a cell.

Error:
Run-time error '-2147417848 (80010108)':
Method `Range of object_'Worksheet' failed

I then get `Not Enough System Resources to Display Completely'

Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)

'Define General Variables
    
    Dim SIZE
    Dim CLASS
    Dim MATERIAL
    Dim PIPETYPE
    Dim GROUP
    Dim SCH
    Dim PRES_D
    Dim TEMP_DU
    
    Dim TEMP_DL
    Dim MATFAM
    Dim GROUP_PT

'Define Pressure/Temperature Calculation Variables

    Dim TEMP_PTL
    Dim TEMP_PTU
    Dim PRES_PTL
    Dim PRES_PTU
    Dim PRES_PTDIFF
    Dim TEMP_PTDIFF
    Dim RATIO_PT
    Dim TEMP_INC 'Temperature Increase above TEMP_PTL
        
'Define Coordinate Calculation Variables

    Dim ROW_S
    Dim COL_S1
    Dim COL_S2
    Dim ROW_PT
    Dim COL_PT1
    Dim COL_PT2

'   Set-up Known Variables
    
    CLASS = Worksheets("ASME B31.3 CALCULATION").Cells(4, "D").Value
    GROUP = Worksheets("ASME B31.3 CALCULATION").Cells(14, "D").Value
    GROUP_PT = GROUP & " - " & CLASS
    
With Application.WorksheetFunction

    ROW_PT = .Match(GROUP_PT, Worksheets("ASME B16.5 TABLES").Range("C3:C142"), 0) 'Determine Row Number for Group
    COL_PT1 = .Match(TEMP_DU, Worksheets("ASME B16.5 TABLES").Range("D2:AF2"), 1) ' Determine Column Number for Lower Temperature
    COL_PT2 = COL_PT1 + 1 ' Determine Column Number for Upper Temperature

    TEMP_PTL = .Index(Worksheets("ASME B16.5 TABLES").Range("D2:AF2"), 1, COL_PT1, 1) ' Determine Lower Temperature Value
    TEMP_PTU = .Index(Worksheets("ASME B16.5 TABLES").Range("D2:AF2"), 1, COL_PT1 + 1, 1) ' Determine Lower Temperature Value

    PRES_PTU = .Index(Worksheets("ASME B16.5 TABLES").Range("D3:AF142"), ROW_PT, COL_PT1) 'Determine Lower Pressure Value
    PRES_PTL = .Index(Worksheets("ASME B16.5 TABLES").Range("D3:AF142"), ROW_PT, COL_PT2) ' Determine Upper Pressure Value

End With

    PRES_PTDIFF = PRES_PTU - PRES_PTL 'Determine Pressure Differential
    TEMP_PTDIFF = TEMP_PTU - TEMP_PTL 'Determine Temperature Differential

    RATIO_PT = PRES_PTDIFF / TEMP_PTDIFF 'Increase in Pressure Per Deg C

    TEMP_INC = TEMP_DU - TEMP_PTL
    
    Sheets("ASME B31.3 CALCULATION").Range("M16") = (PRES_PTU - (RATIO_PT * TEMP_INC))
        
   
End Sub

Any help/pointer would be very much appreciated :-)

Thanks

Steve