If you wants to configure or set drop down list in purchase order voucher,follow these options
Step 1 Open your company
Step 2 Go to administartion and then go to configuration
Step 3 Select voucher series configuration and then select type of voucher purchase order voucher
Step 4 After selecting voucher type,select voucher series and then on the right side select voucher configuration option
Step 5 Now voucher series conguration for purhcase order voucher will be opened
Step 6 Now from options,check if any filter value for party or item dropdown is enabled,if it is not required,you can disable
Step 7 Now save the configuration
Step 8 Now in purhcase order voucher,while item entry,dropdown will be default