Nrthugu

Sub abrirotroworkbook()



    Dim y As Workbook ' a donde se va a pegar

    Dim vals As Variant



    Set y = Workbooks.Open("C:Pathfilename.xlsm")



    'Now, copy what you want from x:



    vals = Sheet12.Range("A3:B3").Value





    'Now, paste to y worksheet:



    y.Sheets("Sheet1").Range(A5).Value = vals





End Sub

When I run this code it saves the information of the book I have opened in the variable vals, then it opens the other workbook, but when it tries to paste in it, the following error appears:

run time error '9' subscript out of range

I don't know what the problem is.

As @Storax said, your variable vals is not opening a workbook but creating an array.
You don't need an array for just copying two cells. You would have to loop through the array. The code below is basic. If you are copying from ThisWorkbook(the workbook your code is in) it will work. If it is another workbook then you can create another variable to open it, and use the variable instead of ThisWorkbook.

Dim wb1 As Workbook 'the source workbook

Set wb1 = Workbooks.Open("C:Pathsourcefilename.xlsm") 'change name and path as required



Dim wb2 As Workbook 'the destination workbook

Set wb2 = Workbooks.Open("C:Pathdestinationfilename.xlsm") 'change name and path as required



wb2.Sheets("Sheet1").Range("A5:B5").Value = ThisWorkbook.Sheets("Sheet12").Range("A3:B3").Value  'replace ThisWorkbook with `wb1`