If you have created a location that you are no longer using then you can hide it form your view.
There’s no way to delete a location/warehouse from Erply. That is because otherwise there wouldn’t be any location to associate the documents that have been made from that warehouse. But for the location to not be visible at all times, as it shouldn’t be in use anymore you can hide it. So you would still have the history of that warehouse, but it will not be visible to users who don’t need to see it.
When you go to Settings - Users - Open a user, then you can see at the bottom left the locations that the user can see. You can take off the tick from in front of the warehouse that is no longer in use. That way the user can’t see the warehouse anymore.
Other possibility is to add “not in use” to your location name and move it below on the list. Open Retail→ Locations→ open a location which is not used anymore, add “not in use” to location name and save. In the end of each line there are arrows, use those to move your location at the end of the list.