Error with find / findfirst to modify existing records on a table (codeunit)

HI everyone.. I'm having a lot of trouble trying to insert records of a vendor in a table.. In this case I already have a record and I need to update (modify) the content but It raises a error that this record already exists.

I was trying to find the existing record making

prodform."Vendor No.":="Source No.";
IF prodform.FIND('=') THEN BEGIN

I also tried just to do

IF prodform.FINDFIRST() THEN BEGIN




But nothing is working.. here's the code and images of the problem... where prodform is a record of "Produtos Fornecedor" table which have the fields Vendor No (supplied by the table Vendor), Data Ultima Compra, Preço, Quantidade, Item No. (linked to Item table) and Vendor Name (flowfield with Lookup(Vendor.Name WHERE (No.=FIELD(Vendor No.))) )

IF "Item Ledger Entry Type"=0 THEN BEGIN
prodform.SETRANGE("Item No.",Item."No.");
prodform.SETRANGE("Vendor No.","Source No.");
Item."Data Ultima Compra":="Posting Date";
Item."Vendor No.":="Source No.";

prodform."Vendor No.":="Source No.";
IF prodform.FIND('=') THEN BEGIN
prodform."Data Ult. Compra":="Posting Date";
prodform.Quantidade:="Invoiced Quantity";
prodform.Preço:=ValueEntry."Cost per Unit";
prodform.MODIFY;
END
ELSE BEGIN
prodform.INIT;
prodform."Vendor No.":="Source No.";
prodform."Data Ult. Compra":= "Posting Date";
prodform.Quantidade:="Invoiced Quantity";
prodform.Preço:=ValueEntry."Cost per Unit";
prodform."Item No.":=Item."No.";
prodform.INSERT;
END;
Item.MODIFY;
END;