Please search the forum before posting. It was already made clear by @Tut in the below post from November 2020 that:
Tut Nov '20
First, you must create inventory locations and assign inventory items to them. See https://www.manager.io/guides/10677 . In general, an inventory location field will then appear after you select an inventory item on a sales or purchase form (receipt, payment, or invoice).
But you can only choose one inventory location per transaction. If you have line items to be sold from or purchased for separate inventory locations, you must enter separate transactions for each location.
Also read the Guide about inventory transfers: https://www.manager.io/guides/10707.