Microsoft launched the new 10080 preview build of Windows 10 for phones Thursday, but this new version of the OS still won't work with some models of the Lumia 640, due to an OS upgrade path issue.
You may remember that the last Windows 10 for phones preview had a similar issue with some Lumia 640 devices, and it appears that same kind of bug is present with the 10080 build. Specifically, if your Lumia 640 has either OS version 8.10.15127.138 or 8.10.15135.146 installed, the new Windows 10 for phones preview cannot be installed.
A rep for Microsoft explained what was going on in a company support message board:
"To get from WP8 up to Win10, there has to be a software update path created. When an update is made available on the server, your on-device OS checks to see where you are currently (what version it has), where you'll be going (what is new on the server), and it validates that there is a valid path between the two. To over-simplify it, think of being in city A (WP 8.1), wanting to go to city B (Win10), and having a map that shows your device how to get there (the update path). The issue for users hitting the current "no build" situation... is that there's no map in place. The software update path from your current OS build up to the new Win10 build is not in place yet."
Microsoft is still working on a solution to this problem but the rep added, "It's going to take a bit of work to complete this." They hope to have an update on this situation sometime early next week
Source: Microsoft Community