I cannot choose the inventory on hand account in purchasing. Before it shows up in the “account” field, I still have to input through the “item” field an inventory item then the system automatically fills up the “account” field with “inventory on hand”.
I cannot choose “inventory on hand” in the assets list.
In connection to this, I don’t know where I should input freight-in. Do I have to create an account for it in the chart of accounts?
That is by design. Inventory on hand requires that you have predefined inventory items. Otherwise the transaction would end up in Suspense. And you cannot buy something for Inventory on hand that is not an inventory item.
For your freight-in question, see:
So, when purchasing multiple inventory items, like this image above, I have to include “freight-in” in my inventory list?
Like this?
Oh. I figured, I can already choose freight-in in the item list when purchasing so I don’t have to enter it in the inventory items. Thanks for your answer! 
@Zab - you seem to be showing screenshots from different versions of Manager.
In your recent post the screenshot has the Account first and the Inventory Item second
In your first post the screenshot has the Inventory Item first and the Account second, this is the latest version.
As you have discovered, Freight-In is a built in default under Item
I got the first screenshot from lubos’ guide to show the situation he presented there. While the second screen shot was from my own laptop. Thanks anyways! 
By the way, as I have tried it, I figured that the freight in is distributed based on cost, isn’t it? Can it be changed like it will be allocated to the inventory items based on weight or a unit measurement?
Then you should be updating your laptop version as you are very out of date.
The Freight-in cost is distributed based on the quantities - so if your quantities are mixed (kgs, ea, cases) then the freight-in code will add up the total quantities and will divide the cost by that total.