Where (what line) is the error?

Also, in most cases, there is no need to activate or select objects. One
can refer to them directly. For example, use

With Worksheets("Per.1 Rate.Pen. nr. 1")
.Cells(16, 2).Value = P1R1_udbt_start
.Cells(30, 6).Value = P1R1_udbt_length
.Cells(31, 8).Value = P1R1_amount_primo
End With

There are other improvements possible but focus on the immediate error
first.

Also, I suspect that even after you fix the current problem the code will
not do what you want. The Cells(10 + P1R1_udbt_length, 12).Select appears
to be an attempt to position yourself at the next empty cell but then the
Cells(10 + i, 1).Select and other processing in the loop will overwrite the
data written in an earlier iteration of the loop.

--
Regards,

Tushar Mehta
www.tushar-mehta.com
Excel, PowerPoint, and VBA add-ins, tutorials
Custom MS Office productivity solutions

In article ,
says...
Public P1R1_udbt_start As Integer, P1R1_udbt_length As Integer,
P1R1_amount_primo As Double

Public Sub Export_P1R1()

Dim i As Integer, j As Integer
Dim sheets As worksheets
Dim worksheets As worksheets

worksheets("Per.1 Rate.Pen. nr. 1").Activate
With Cells
Set Cells(16, 2) = P1R1_udbt_start
Set Cells(30, 6) = P1R1_udbt_length
Set Cells(31, 8) = P1R1_amount_primo
End With

sheets("IndtPer1").Activate

For i = 0 To 55
On Error Resume Next

With Cells
Cells(10 + i, 1).Select
If Cells(10 + i, 1) = P1R1_udbt_start Then
Cells(10 + i, 12).Select
End If

For j = 0 To P1R1_udbt_length - 1
Cells(10 + i + j, 12) = P1R1_amount_primo
Next j
Cells(10 + P1R1_udbt_length, 12).Select

End With

Next i

sheets("Per.1 Rate.Pen. nr. 1").Select

End Sub