Set rangePartner = Range(Partner.Cells(2, 2), Partner.Cells(2, 2).End(xlDown)) Needs an explicit parent object for the Range otherwise it uses the active sheet which will cause problems if that sheet is not Partner, since the cells use Partner.
Your With is currently doing nothing, you need a . infront of cells.
LookIn:=Values should be xlvalues
Don't use a Range method or property in the same line as Find if Find fails then the program will error since it returns an empty object that has no properties or methods.
I don't see a value for partnerid.
All together something like this:
Dim Partner As Worksheet
Dim rangePartner As Range
Set Partner = Worksheets("Partner")
Set rangePartner = .Range(.Cells(2, 2), .Cells(2, 2).End(xlDown))
Dim partnerid As String 'This still needs a value assigned
Dim segment As Range
Set segment = .Cells.Find(partnerid, LookIn:=xlValues, SearchOrder:=xlByRows)
If Not segment Is Nothing Then
Set segment = segment.Offset(0, 3)