Congrats on starting to use VBA. There's several things in your code that could use improvement. You want to avoid using select (a common beginner task). You also don't even need to move around your sheet, or even use copy/paste.
However, see below where I've broken up your code with some statements to stop and check where you're at. I think this will accomplish what you want, but also help you gain a better grasp of what you're doing (it's always a battle getting started!)
Keep battling.
Sub adfa()
Const turnOnStops As Boolean = True 'change to true or false to review code
Dim WS_Pull As Worksheet:
Set WS_Pull = ActiveSheet 'better to define this with actual sheet name
Dim lastrow As Long:
lastrow = WS_Pull.Cells(Rows.Count, 1).End(xlUp).Row 'this assumes column a has the bottom row and no rows hidden
If turnOnStops Then
Debug.Print "Lastrow is " & lastrow
Stop
End If
Dim lastcol As Long:
lastcol = WS_Pull.Cells(1, Columns.Count).End(xlToLeft).Column 'same assumptions but with columns on row 1 instead of columna a
If turnOnStops Then
Debug.Print "lastcol is " & lastcol
Stop
End If
Dim myarray As Range:
Set myarray = WS_Pull.Range("A1").Resize(lastrow, lastcol) ' I'm not sure what you're trying to do here.
If turnOnStops Then
Dim theAnswer As Long
theAnswer = MsgBox("The address of myArray is " & myarray.Address & ". Stop code?", vbYesNo)
If theAnswer = vbYes Then Stop
End If
Dim WS_paste As Worksheet: Set WS_paste = Sheets("MRG") 'it would be better to use the SHEET (shown in the VBA project)
WS_Pull.Range("A1", myarray).Copy '<--- what are trying to copy.
If turnOnStops Then
theAnswer = MsgBox("The area copied was " & WS_Pull.Range("A1", myarray).Address & ". Stop code?", vbYesNo)
If theAnswer = vbYes Then Stop
End If
If turnOnStops Then
theAnswer = MsgBox("The area you are going to paste to is " & _
WS_paste.Cells(1, Rows.Count).End(xlUp).Offset(2, 0).Address & " stop code?", vbYesNo)
If theAnswer = vbYes Then Stop
End If
End Sub